Basic Introduction to Film Capacitors

Catalog

I Basic structure

The internal structure of film capacitors is mainly as follows: metal foil (or a foil obtained by metalizing plastic) is used as the electrode plate, and plastic is used as the dielectric. Obtained by winding or stacking process. The different arrangements of foils and films lead to a variety of construction methods. The figure below is a typical schematic diagram of thin-film capacitors.

typical schematic diagram of thin-film capacitors

The film capacitor is a capacitor with a metal foil as an electrode, and a plastic film such as polyethylene, polypropylene, polystyrene, or polycarbonate, which is overlapped from both ends and wound into a cylindrical structure. And according to the type of plastic film, they are also called polyethylene capacitors (also known as Mylar capacitors), polypropylene capacitors (also known as PP capacitors), polystyrene capacitors (also known as PS capacitor), and polycarbonate capacitors.

II Basic characteristics

film capacitor

The film capacitor has many excellent characteristics. Its main equivalences are as follows: non-polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and low dielectric loss. Based 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 extremely low dielectric loss must be used to ensure that the signal is transmitted without too much distortion. Among all plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most remarkable characteristics.

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

Among all the plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most remarkable characteristics. Of course, the prices of these two capacitors are relatively high. However, in recent years, in order to improve the sound quality of audio equipment, the parts materials used have become more and more advanced, and the price 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 the equipment of the XYZ brand, claiming how many PP-type capacitors or PS-quality capacitors of XYZ brand are used as an endorsement in sound quality, the reason is here.

Summary of characteristics: The capacity range of the film capacitor is 3pF-0.1μF, the DC working voltage is 63-500V, suitable for high frequency and low frequency, and the leakage resistance is greater than 10000 Ω.

III Metalized film capacitors

metalized film capacitors

The usual film capacitors are manufactured by overlapping and winding metal foils such as aluminum which is used as electrodes and plastic films together. But there is another manufacturing method for film capacitors, called Metallized Film. Its manufacturing method is to vacuum-deposit a thin layer of metal on the 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 a thin film capacitor into a capacitor with a small capacity and a large capacity. For example, the common MKP capacitor is the name of Metalized Polypropylene Film Capacitor, while MKT is the name of Metalized Polyester Capacitor.

The films used for metalized film capacitors include polyethylene, polypropylene, polycarbonate, etc. In addition to the wound type, there are also laminated types. Capacitors of this type of metalized film have a so-called self-healing action, that is, if the small part of the electrode is short-circuited due to the weak electrical quality, due to the electrostatic energy or short-circuit current carried by the capacitor at that time, a larger area of melting and evaporation was induced to restore the insulation, and the capacitor was restored to the role of the capacitor again.

Characteristics of metalized film capacitors

The metalized film capacitor is to deposit a metal film on the surface of the polyester film instead of metal foil as the electrode. Because the thickness of the metalized film layer is much smaller than the thickness of the metal foil, the volume after winding is also much smaller than the volume of the metal foil capacitor. The biggest advantage of metalized film capacitors is the "self-healing". The so-called self-healing property is that if the thin film medium has defects at a certain point and a breakdown short circuit occurs under the action of overvoltage, the metallization layer of the breakdown point can melt and evaporate instantly under the action of the arc to form a small metal-free zone. The two-pole pieces of the capacitor are re-insulated from each other and can still continue to work, thus greatly improving the reliability of the capacitor. From the principle analysis, there should be no short-circuit failure mode for metalized film capacitors, and metal foil capacitors will have many short-circuit failure phenomena. Although metalized film capacitors have the above-mentioned huge advantages, compared with metal foil capacitors, they also have the following two disadvantages:

First, capacity stability is not as good as foil capacitors. This is because metalized capacitors are prone to capacity loss and self-healing in long-term working conditions, which can lead to capacity reduction. Therefore, if used in an oscillation circuit that requires high capacity stability, Metal foil capacitors should be used better.

Another major disadvantage is its poor ability to withstand large currents. This is because the metalized film is much thinner than the metal foil, and its ability to carry large currents is weak. In order to improve the metalized film capacitors, the main improvement methods of capacitor products are: Use double-sided metalized film as an electrode; Increase the thickness of the metalized coating; Improved metal welding process on the end face to reduce contact resistance.

IV Precautions for using film capacitors

1. Working voltage

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

2. Working current

The pulse (or AC) current through the capacitor is equal to the product of the capacitance C and the rate of voltage rise, that is, I = C × dV / dt. Due to the loss of the capacitor, when you use it under high frequency or high pulse, the pulse (or AC) current through the capacitor will cause the capacitor to heat itself, which will cause thermal breakdown (smoke, fire). Therefore, the safe use conditions of capacitors are not only limited by the rated voltage, but also by the rated current.

Rated current is considered to be composed of pulse current (peak current, which is limited by dV / dt index) and continuous current (expressed in peak-to-peak value or effective value), which is determined by breakdown mode.

For capacitors used under high frequency or high pulse conditions, we recommend polypropylene film capacitors or CL23B laminated capacitors. CBB81 / A / B series polypropylene film high-voltage capacitors give a rated peak-to-peak current at a duty ratio (DUTY) of 15%. CBB21 / A / B series S correction capacitors are rated for peak-to-peak current. Under normal circumstances, it is not allowed to exceed the rated peak-to-peak current.

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

3. The effective value conversion relationship of various waveforms

relationship of various waveforms 

4. Capacitor for suppressing electromagnetic interference of power supply

4.1 When using capacitors in power supply cross-circuit circuits to eliminate noise, not only normal voltage but also abnormal pulse voltage (such as lightning) may occur, which may cause the capacitor to smoke or fire. Therefore, the safety standards of cross-line capacitors have strict regulations in different countries. It is not allowed to use DC capacitors as jumper capacitors.

4.2 Class X capacitors for power supply electromagnetic interference suppression

It is suitable for occasions that will not cause electric shock hazards when the capacitor fails, and it is used to suppress differential mode interference. Class X capacitors are divided into three categories X1, X2, X3 (see table below).

Class X capacitors 

4.3 Class Y capacitors for power supply electromagnetic interference suppression

It is suitable for occasions where capacitor failure will cause an electric shock hazard. 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 capacitor

5. Charge and discharge of the capacitor

Since the charge and discharge current of the capacitor depends on the product of the capacitance and the rate of voltage rise, even low-voltage charge and discharge may produce a large instantaneous charge and discharge current, which may cause damage to the performance of the capacitor, such as short circuit or open circuit. When charging and discharging, please connect a current-limiting resistor of 20Ω/ V to 1,000Ω/ V in series to limit the charge and discharge current within the specified range.

When multiple thin-film capacitors are connected in parallel for withstand voltage test or life test, please connect a current limiting resistor of 20Ω/ V to 1 000Ω/ V or higher for each capacitor in series.

6. Flame-retardant

Although a flame-retardant material-flame-retardant epoxy resin or plastic shell is used in the outer packaging of the film capacitor, the continuous high temperature or flame outside can still deform the capacitor core and cause the outer package to crack, causing the capacitor core melting or burning.

7. Surface temperature rise (△ T)

7.1 When the capacitor is used in AC and pulse occasions, the current flowing through the capacitor makes it 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 cannot exceed the maximum value specified in the product catalog and it is particularly necessary to monitor the temperature rise of the capacitor when it is loaded.

7.2 The method of measuring the temperature rise of the capacitor surface is shown in the Figure below. The tested capacitor must be applied with working AC, pulse voltage, and working frequency.

capacitor test

8. Noise

The humming sound of the capacitor is the sound produced by the vibration of the capacitor film due to the Coulomb force of the two opposite electrodes. The more severe the distortion of the voltage and frequency waveform through the capacitor, the greater the humming. But this buzzing sound will not cause any damage to the capacitor.

9. Storage environment requirements

1. Due to the existence of hydrochloride, hydrosulfide, sulfuric acid, etc. in the atmosphere, the product is stored in the atmosphere, and it must be noted that the solderability of the lead-out terminal will deteriorate.

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

Temperature: no more than 35°C

Humidity: no more than 80% RH

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

For bulk products, no more than 24 months.

For diameter-knitted products, no more than 12 months.

3. Storage requirements for SMD products

When the Class 1 MBB (Moisture Barrier Bag) is not opened and stored at a temperature of less than 35°C and a humidity of less than 80% RH, a storage period of 12 months can be guaranteed. When the MBB is turned on, it can be guaranteed for 168 hours when the temperature is less than 30°C and the humidity is less than 60% RH. If you unpack the remaining unused products, we recommend reusing the same MBB packaging or controlling the humidity and temperature of the storage environment.

*Utmel Electronic has a large number of film capacitors in stock, you can check it here, or send us your RFQ.

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