The article analyzes the capacitance and electrolytic capacitance of PCB technology

- Aug 03, 2018-

Capacitors can be generally divided into ordinary capacitors without polarity and electrolytic capacitors with polarity.

Common capacitors are fixed capacitors, semi-adjustable capacitors (fine tuning capacitors) and variable capacitors.

一文解析PCB技术电容和电解电容

A.

Fixed capacitor: a capacitor whose capacitance cannot be changed after being made.


1.

Capacitance classification: capacitors are generally classified by the medium.


1) paper capacitor: generally, the capacitance is between pF and uF, and the pressure resistance is 250V, 400V, 630V, etc. The capacitance error is generally: plus or minus 5%, plus or minus 10%, plus or minus 20%.

The other is a metallized paper capacitor with a limited self-healing capability.

Generally cannot be used in high frequency circuits, the operating frequency is only a few tens of KHZ.


2) polyester capacitor:


3) polystyrene capacitor:


4) polypropylene capacitor:


5) ptfe capacitor:


6) polyimide film capacitor:


7) polycarbonate film capacitor:


8) composite film capacitor:


9) paint film capacitor:


10) laminated metallized polycarbonate capacitor:


11) mica capacitor:


12) porcelain dielectric vessel: low price and wide application.

It can be divided into two types: low voltage low power and high voltage high power.


Low-voltage and low-power porcelain dielectric vessels can be divided into I and II types according to the properties and characteristics of the materials used.

The characteristic of type I is low loss of medium, good stability of electric capacity for temperature, frequency, voltage and time.

The type II is characterized by small size, poor stability and large medium loss, and is often used in low frequency circuits.

Ultra-high frequency porcelain dielectric vessel can be used in high frequency circuits with a frequency not exceeding 500MHZ.


High voltage and high power porcelain dielectric vessels are usually only suitable for use in low loss and low power circuits.


13) glass-glaze capacitor:


2.

Model name of capacitor:


1) the models of capacitors in various countries are not unified. The naming of domestic capacitors consists of four parts:


Part 1: use letters for names, capacitors are C.


Part two: use letters to represent materials.


Part three: use Numbers to represent the classification.


Part four: use Numbers for Ordinal Numbers.


2) marking method of capacitance:


(1) direct labeling method: use letters and Numbers to mark models and specifications directly on the shell.


(2) literal symbol method: the capacity is expressed by the regular combination of Numbers and characters.

A unit of electrical capacity: P, N, u, m, F, etc.

It's the same thing as the resistance.

The nominal allowable deviation is the same as the resistance.

For capacitors less than 10pF, the allowable deviation shall be replaced by letters: B - - - - - 0.1pF, C - - - - - 0.2pF, D - - - - - - 0.5pF, F - - + 1pF.


(3) color marking method: the same as the resistance expression method, the unit is generally pF.

The resistance of small electrolytic capacitors is also color-coded. The position is close to the root of the positive electrode lead line, and the meaning is shown in the following table:


The color is black, brown, red, orange, yellow, green, blue, purple and gray


Pressure resistance 4V 6.3V 10V 16V 25V 32V 40V 50V 63V


(4) marking method for imported capacitors: generally, imported capacitors consist of 6 components.


Item 1: alphabetize the categories:


Item 2: use two digits to represent its shape, structure, encapsulation mode, start of lead and its relation to axis.


Item 3: temperature characteristics of temperature compensated capacitors with useful letters and colors, whose meanings are shown in the following table:


The color temperature coefficient of the ordinal letter allows deviation from the color temperature coefficient of the letter


1 A gold +100 R yellow -220


B grey plus 30 S green minus 330


3 C black 0 T blue minus 470


4 G plus or minus 30 U purple minus 750


5 H brown -30 plus or minus 60 V -1000


6 J plus or minus 120 w-1500


7 K plus or minus 250 X minus 2200


8 L red -80 plus or minus 500 Y -3300


9 M plus or minus 1000 Z minus 4700


10 N plus or minus 2500 SL +350~-1000


P orange -150 YN -800 -5800


Note: the temperature coefficient of the unit 10 e - 6 / ℃;

The allowable deviation is %.


Item 4: use Numbers and letters for pressure resistance, letters for valid values, and Numbers for the power of 10 of the multiplicand.


Item 5: nominal capacity, represented by three digits, the first two are valid values, and the third is a power of 10.

When you have a decimal, it's R or P.

The unit of an ordinary capacitor is pF, and the unit of an electrolytic capacitor is uF.


Item 6: allowable deviation.

In one letter, the meaning is the same as that of domestic capacitors.

There is also the use of color marking method, meaning and domestic capacitor marking method the same.


3.

Main characteristic parameters of capacitance:


(1) capacity and error: the maximum allowable deviation range of actual capacity and nominal capacity.

It is generally divided into 3 levels: level I + 5%, level II + 10%, level III + 20%.

In some cases, there is a level 0, with an error of plus or minus 20%.

The allowable error of precision capacitor is small, while the error of electrolytic capacitor is large.


Commonly used capacitors have the same precision grade as resistors.

In alphabetical form: D -- 005 -- plus or minus 0.5%;

F - 01 level - plus or minus 1%;

G - grade 02 - plus or minus 2%;

J - grade I - plus or minus 5%;

K - class II - plus or minus 10%;

M - level III - plus or minus 20%.


(2) rated working voltage: capacitor can work stably and reliably in the circuit for a long time, and the maximum dc voltage, also called withstand voltage.

For devices with the same structure, medium and capacity, the higher the pressure resistance, the larger the volume.


(3) temperature coefficient: in a certain temperature range, temperature per 1 ℃, relative change of capacitance value.

The smaller the temperature coefficient, the better.


(4) insulation resistance: used to indicate the size of leakage.

Capacitors of generally small capacity, high insulation resistance, in the hundreds or thousands of megaohms.

The insulation resistance of electrolytic capacitors is generally small.

Relatively speaking, the greater the insulation resistance, the better, less leakage.


(5) loss: under the action of electric field, the energy consumed by the capacitor during unit time of heating.

These losses mainly come from medium loss and metal loss.

It is usually expressed as the loss Angle tangent value.


(6) frequency characteristic: the property that the electrical parameters of the capacitor change with the electric field frequency.

Capacitors operating at high frequencies have reduced capacitance because the dielectric constant is smaller at high frequencies than at low frequencies.

The loss also increases with the increase of frequency.

In addition, when working at high frequency, the distribution parameters of capacitor, such as electrode resistance, lead wire and interelectrode resistance, electrode inductance and lead wire inductance, will affect the performance of capacitor.

All of which limits the frequency at which capacitors can be used.


Different types of capacitors, the highest use frequency is different.

Small mica capacitors within 250MHZ;

The wafer type ceramic dielectric vessel is 300MHZ.

Round tube porcelain dielectric vessel is 200MHZ.

Disc type porcelain interface up to 3000MHZ;

The small paper capacitor is 80MHZ.

Medium sized paper capacitor has only 8MHZ.


4.

Circuit symbol:


5.

Use of capacitors:


1) select the appropriate model:


2) reasonably determine the accuracy of the capacitor:


3) determine the rated working voltage of the capacitor: for general circuits, the working voltage of the circuit shall be 10%~20% of the rated voltage of the capacitor;

When there is a pulse voltage, the working voltage shall be the maximum voltage of the pulse.

When applied to ac, the rated voltage increases with increasing frequency.

When the temperature environment is high, the rated voltage should be larger.


4) choose capacitors with high insulation resistance as far as possible:


5) consider the temperature coefficient and frequency characteristics:


6) pay attention to the use environment:


2.

Electrolytic capacitor:


Electrolytic capacitors generally have positive and negative poles, that is, have polarity.

Therefore, when used in the circuit, the positive and negative terminals cannot be connected incorrectly.

It is now possible to manufacture electrolytic capacitors that have no polarity or are used in ac circuits, called bipolar capacitors or non-polar capacitors.

Under the action of external voltage, the device that causes partial damage for some reason has the function of self-repairing, which is called self-healing of electrolytic capacitor.


Normally only in the electrolytic capacitor - 20 ℃ ~ + 70 ℃ within the scope of use.

The characteristic is greatly influenced by temperature and frequency.

Aluminum electrolytic capacitors are commonly used.

The model of aluminum electrolytic capacitor is generally CDXX, and its capacity, pressure resistance and positive and negative terminals are marked on the shell.

Sometimes the length of the lead is also used to indicate that the long line is positive and the short line is negative.


The loss of aluminum electrolytic capacitors is large, and the characteristics of temperature and frequency are poor, which limits the application in ac circuit.


3.

Tantalum electrolytic capacitor:


Tantalum electrolytic capacitors are capacitors with metal tantalum as electrode and tantalum oxide as medium.

Characteristics: high chemical stability, high rated pressure resistance, high temperature resistance, high mechanical strength, small volume.

Commonly used CA mark, its capacity ranges from 0.47uF to 1000uF, and the rated pressure resistance mainly includes 6.3v, 10V, 16V and 63V.

The performance is much better than aluminum electrolytic capacitors, but the price is more expensive.


Other electrolytic capacitors include niobium electrolytic capacitors with better performance and smaller volume.

Titanium electrolytic capacitor, tantalum-niobium electrolytic capacitor.


Four.

Miniature capacitor:


1.

Chip ceramic capacitor: that is, surface - mounted capacitor.

The capacity range is 1pF ~ 47800pF, and the pressure resistance is 25V and 50V.

The size of package installation is the same as the meter resistance.


2.

Chip tantalum capacitor: small volume, large capacity.

Welding temperature limit under 280 ℃, time less than 10 s.

Its capacity range is 0.1uF ~ 100uF, dc working voltage is 4 ~ 50V, commonly used withstand voltage is 16V, 35V, etc.


3.

Double layer electrolytic capacitors: between batteries and capacitors, with a large capacity, known as supercapacitors.

Generally, it can only be used in dc or low frequency conditions and is often used as an energy storage element.


4.

Semiconductor capacitor