Basic introduction to film capacitors

Mondo Finance Updated on 2024-02-02

Film capacitors are also known as plastic film capacitors. It uses a plastic film as a dielectric. Depending on the medium, there are many types of capacitors, such as electrolytic capacitors, paper capacitors, film capacitors, ceramic capacitors, mica capacitors, and air capacitors. Film capacitors are widely used in analog signal cross-linking, power supply noise bypass (reverse cross-linking) and other occasions.

Directory.

The internal structure of a film capacitor is mainly as follows: a metal foil (or a foil obtained by metallizing plastic) is used as an electrode plate, and plastic is used as a dielectric. It is obtained by a winding or stacking process. The different arrangement of foils and films has led to a variety of construction methods. The figure below is a typical schematic diagram of a film capacitor.

Typical schematic diagram of a film capacitor.

Film capacitors are capacitors with metal foil as electrodes, and plastic films such as polyethylene, polypropylene, polystyrene or polycarbonate are superimposed and wound from both ends to form a cylindrical structure. Depending on the type of plastic film, they are also known as polyethylene capacitors (also known as polyester capacitors), polypropylene capacitors (a.k.a. PP capacitors), polystyrene capacitors (a.k.a. PS capacitors), and polycarbonate capacitors.

Film capacitors.

Film capacitors have many excellent properties. The main equivalents are as follows: non-polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and low dielectric lossBased on the above advantages, film capacitors are widely used in analog circuits. Especially in the part where the signal is connected, capacitors with good frequency characteristics and very low dielectric losses must be used to ensure that the signal is transmitted without too much distortion. Of all plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most significant characteristics.

Its structure is the same as that of paper, and the dielectric capacitor medium is polyester or polystyrene. Polyester film capacitors have the characteristics of high dielectric constant, small size, large capacity and good stability. They are suitable for use as bypass capacitors. Polystyrene film capacitors have low dielectric loss and high insulation resistance, but have a large temperature coefficient and can be used in high-frequency circuits.

Of all plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most significant characteristics. Of course, the ** of these two capacitors is relatively high. However, in recent years, in order to improve the sound quality of audio equipment, the materials used for components have become more and more advanced, and ** is not the most important consideration. In recent years, the frequency and number of PP capacitors and PS capacitors used in audio equipment have also increased. Readers can often see XYZ brand devices, claiming how many XYZ brand PP capacitors or PS-quality capacitors are used as endorsements for sound quality, and here's why.

Summary of characteristics: Film capacitors have a capacitance range of 3pf-01 F, DC working voltage is 63-500V, suitable for high and low frequency, leakage resistance is greater than 10000.

Metallized film capacitors.

Usually film capacitors are made by overlapping and winding together with a metal foil such as an aluminum-plastic film used as an electrode. But there is another manufacturing method for film capacitors called metallized film. It is manufactured by vacuuming a thin layer of metal on a plastic film as an electrode. In this way, the thickness of the electrode foil can be omitted, and the volume per unit capacity of the capacitor can be reduced. Therefore, it is easier to make film capacitors into small and large capacitances. For example, a common MKP capacitor is the name of a metallized polypropylene film capacitor, while MKT is the name of a metallized polyester capacitor.

Films used for metallized film capacitors include polyethylene, polypropylene, polycarbonate, etc. In addition to the winding type, there is also a laminated type. This metallized film capacitor has a so-called self-healing effect, that is, if a small part of the electrode is short-circuited due to weak electrical quality, it is due to the electrostatic energy or short-circuit current carried by the capacitor at that time, which restores the insulation by inducing melting and evaporation over a large area, and the capacitor resumes to the role of a capacitor again.

Characteristics of metallized film capacitors.

Metallized film capacitors are deposited with a metal film on the surface of the polyester film, rather than using the metal foil as the electrode. Since the thickness of the metallized film layer is much smaller than that of the metal foil, the volume after winding is also much smaller than that of the metal foil capacitor. The biggest advantage of metallized film capacitors is that they are "self-healing". The so-called self-healing property is that if the film medium is defective at a certain point and a breakdown short circuit occurs under the action of overvoltage, the metallized layer at the breakdown point can immediately melt and evaporate under the action of the arc, forming a small metal-free zone. The two poles of the capacitor are re-insulated from each other and can still continue to operate, greatly improving the reliability of the capacitor. From the perspective of principle analysis, metallized film capacitors should not have short-circuit failure modes, and metal foil capacitors will have many short-circuit fault phenomena. While metallized film capacitors have the great advantages mentioned above, they also have two disadvantages compared to metal foil capacitors:

First of all, capacitance stability is not as good as that of foil capacitors. This is because metallized capacitors are prone to capacitance loss and self-healing under long-term operating conditions, which can lead to capacitance reduction. Therefore, if it is used in oscillation circuits that require high capacitance stability, metal foil capacitors should be used preferably.

Another major drawback is its poor ability to withstand large currents. This is because metallized films are much thinner than metal foils, and their ability to carry large currents is weak. In order to improve metallized film capacitors, the main improvement methods of capacitor products are: using double-sided metallized film as electrodes; increase the thickness of the metallized coating; Improved end-face metal welding process to reduce contact resistance.

The choice of film capacitor depends on the highest voltage applied and is influenced by factors such as the applied voltage waveform, current waveform, frequency, ambient temperature (capacitor surface temperature), and capacitance. Before use, check whether the voltage waveform, current waveform, and frequency (in the case of high frequency, the permissible voltage varies with the capacitor type, please refer to the manual for details) are within the rated value.

The impulse (or AC) current through the capacitor is equal to the product of the capacitor c and the voltage rise rate, i.e., i = c & times; dv/dt。Due to the loss of capacitors, when you use a capacitor at high frequencies or high pulses, the pulsed (or AC) current through the capacitor will cause the capacitor itself to heat up, resulting in thermal breakdown (smoke, fire). Therefore, the safe conditions for the use of capacitors are limited not only by the rated voltage, but also by the rated current.

The rated current is considered to consist of a pulse current (peak current, limited by the dv dt index) and a continuous current (expressed as a peak-to-peak or RMS value), determined by the breakdown mode.

For capacitors used in high-frequency or high-pulse conditions, we recommend polypropylene film capacitors or CL23B laminated capacitors. CBB81 A B series polypropylene film high voltage capacitors provide a peak current rating with a duty cycle of 15%. CBB21 A B Series S calibration capacitors are rated for peak-to-peak current. Under normal circumstances, the rated peak-to-peak current is not allowed.

When the actual operating current waveform is different from the given waveform, in general, a polyester film capacitor is used when the temperature rise of the polyester film capacitor itself is 10°C or less, and the temperature rise of the polypropylene film itself is 5°C or less. The surface temperature of the capacitor must not exceed the upper limit of the rated temperature.

The relationship between the various waveforms.

4.1When using capacitors to eliminate noise in power crossover circuits, not only normal voltages may occur, but also abnormal impulse voltages (such as lightning) may occur, causing the capacitors to smoke or **. As a result, safety standards for cross-wire capacitors are strictly regulated in different countries. DC capacitors are not allowed to be used as jumper capacitors.

4.2Class X capacitors for power supply electromagnetic interference suppression.

It is suitable for applications where there is no risk of electric shock in the event of a capacitor failure, and is used to suppress differential mode interference. Class X capacitors are divided into three categories: X1, X2, and X3 (see the table below).

Class X capacitors.

4.3. Class Y capacitors for electromagnetic interference suppression in power supplies.

It is suitable for applications where a failure of a capacitor can lead to a risk of electric shock. It is used to suppress common-mode interference and can be grounded. Class Y capacitors are divided into four categories, such as Y1, Y2, Y3, Y4 (see table below).

Class Y capacitors.

Since the charge-discharge current of a capacitor depends on the product of the capacitor and the rate of voltage rise, even low-voltage charge-discharge may produce large instantaneous charge/discharge currents, which may damage the performance of the capacitor, such as a short circuit or an open circuit. When charging and discharging, connect a current relay resistor in series from 20 V to 1000 V to limit the charging and discharging currents to the specified range.

When multiple film capacitors are connected in parallel for hipot testing or lifetime testing, connect a current-limiting resistor of 20 V to 1000 V or higher for each capacitor connected in series.

Despite the use of flame-retardant materials – flame-retardant epoxy resin or plastic housings – in the outer packaging of film capacitors, sustained external high temperatures or flames can deform the capacitor core, causing the outer packaging to crack and cause the capacitor core to melt or burn.

7.1When a capacitor is used in AC and pulse applications, the current flowing through the capacitor causes it to generate heat. If the heat is too high, it will cause the capacitor to short circuit or even burn out. Therefore, the current flowing through the capacitor must not exceed the maximum value specified in the catalog, and it is especially necessary to monitor the temperature rise of the capacitor while the capacitor is loaded.

7.2. The method of measuring the temperature rise on the surface of the capacitor is shown in the figure below. The capacitor under test must use operating alternating current, impulse voltage, and operating frequency.

Capacitor test.

The humming sound of a capacitor is the sound produced due to the Coulomb force of two opposite electrodes causing the capacitor membrane to vibrate. The more severe the distortion of the voltage and frequency waveforms passing through the capacitor, the louder the hum. But this buzzing sound does not cause any damage to the capacitor.

1.Due to the presence of hydrochloride, hydrogen sulfide, sulfuric acid, etc. in the atmosphere, the product is stored in the atmosphere, and it is necessary to pay attention to the deterioration of the solderability of the lead terminals.

2.The product cannot be exposed to high temperatures and humidity and must be stored in the following environment: (on the basis of not opening the original packaging).

Temperature: not exceeding 35°C (Celsius).

Humidity: The relative humidity does not exceed 80%.

Storage time (from date on product packaging or product body):

For bulk products, no more than 24 months.

Diameter knitted products not exceeding 12 months.

3.Storage requirements for SMD products.

A 12-month storage period is guaranteed when Class 1 MBB (moisture bag) is unopened and stored at a temperature below 35°C and a humidity below 80% RH. When MBB is on, when the temperature is below 30°C and the humidity is below 60%RH, it can guarantee 168 hours. If you open the packaging of the remaining unused product, we recommend reusing the same MBB packaging or controlling the humidity and temperature of the storage environment.

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