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With 3000 words, 11 pictures, take you to know the capacitor!
20201205 03:03:54 【Remember sincerity】
I don't say much nonsense , Go straight to the point .
brief introduction
A highend smartphone has 800~1000 Chip multilayer ceramic capacitors , Capacitors are also used most in general circuits , It can be seen that the function of capacitance is great , This paper mainly introduces some basic knowledge of capacitance .
Two conductors close to each other , A dielectric interlayer that is not conductive , This makes up the capacitor . Capacitors have two very important characteristics , No sudden change of voltage is allowed , So it's often used in filter circuits 、 Bootstrap circuit 、 In circuits such as tuning .
The basic unit of capacitance is F（ Farah ）, The common unit is uF、nF and pF, The conversion relation is ：1uF=1000nF=1000000pF
because F The unit is larger ,1F=1000000uF, Generally do not use .
Naming rules
There are many manufacturers of capacitors , Different manufacturers have different naming rules , Let's take a look at Murata's naming rules .
Such as GRM representative ：Chip Multilayer Ceramic Capacitors for General Purpose.
18 For size ：1.6x0.8mm, English 0603 encapsulation .
In this part, you can check the model selection manual of the corresponding manufacturer .
Series parallel calculation formula
Capacitance is the same as resistance , It can also be connected in series or in parallel .
Two capacitors C1 and C2 parallel connection , The equivalent capacitance is ：
$C_{eq}=C1+C2$
If C1 Far greater than C2, The equivalent capacitance can be reduced to ：
$C_{eq}=C1+C2≈C1$
Two capacitors C1 and C2 Series connection , The equivalent capacitance is ：
$C_{eq}=C1+C2C1C2 $
If C1 Far greater than C2, The equivalent capacitance can also be reduced to ：
$C_{eq}=C1+C2C1C2 ≈C1C1C2 =C2$
It can be seen from the above formula that , The series and parallel connection of capacitors is the opposite of resistance .
The formula for flat plate capacitors
Capacitors store electricity and energy （ Electric potential energy ） The elements of , The formula of flat plate capacitor is ：
$C_{eq}=U_{A}−U_{B}Q =4πkdξ_{r}S $
among $Q$ The charge stored for a capacitor ,$U_{A}−U_{B}$ Is the potential difference between the two plates ,$ξ_{r}$ Relative permittivity ,$S$ It is the area opposite to the plate ,$d$ Is the distance between the two plates . Larger area , The smaller the distance , The larger the volume value .
Standard and common volume values
The standard capacitance of a capacitor is in accordance with E6,E12,E24 Numerical standards , as follows ：
 E6 Series values ：1.0、1.5、2.2、3.3、4.7、6.8 multiply 10 Of n Power ;
 E12 Series values ：1.0、1.2、1.5、1.8、2.2、2.7、3.3、3.9、4.7、5.6、6.8、8.2 multiply 10 Of n Power ;
 E24 Series values ：1.0、1.1、1.2、1.3、1.5、1.6、1.8、2.0、2.2、2.4、2.7、3.0、3.3、3.6、3.9、4.3、4.7、5.1、5.6、6.2、6.8、7.5、8.2、9.1 multiply 10 Of n Power ;
In everyday circuit design , We often use the capacity of ：
 frequentlyused pF Level ：39、43、47、51、56、100、150、200、220、270、300、330、390、470、560、680 etc. .
 frequentlyused nF Level ：1、1.2、1.5、2.2、2.7、3.3、10、22、33 etc. .
 frequentlyused uF Level ：0.1、0.15、0.22、0.33、0.47、1、2.2、10uF etc. .
Capacitance classification
There are many kinds of capacitors , According to the material , The commonly used ones can be divided into the following , The tantalum electrolytic capacitor is generally called tantalum capacitor .
The following is the characteristic comparison of several capacitors given by Murata .
According to the different characteristics of capacitance , The use of the occasion is not the same .
 Ceramic capacitors are small in size （ The smallest 01005 encapsulation ）, Nonpolarity , Cheapness ,ESR low （ Lower than tantalum and aluminum electrolytic capacitors ）, The disadvantage is that the capacity is not as large as electrolytic capacitance , Poor toughness , The impact is fragile , Generally used in small consumer electronic products , Such as mobile phone .
 The electrolytic capacitor has a large capacity , Large size , The price is relatively cheap , It's mostly used in power supply and automotive electronics ;
 Tantalum has a large capacitance , Small volume , Compared with aluminum electrolytic capacitor, its stability is higher , It's mostly used in power supply and automotive electronics ;
 High withstand voltage of thin film capacitor , Good frequency characteristic , High voltage applications are preferred .
Ceramic capacitor
according to EIA and IEC Recommended standards , Ceramic capacitors can be divided into two categories ,Class1 and Class2, That we use a lot X5R and X7R All belong to Class2;
Class1 and Class The dielectric materials are different , Murata gives two contrasts .
C0G It's warm tonic , You can see that as the temperature changes , The rate of change of electrostatic capacity is almost 0
High inductivity series capacitors (B/X5R、R/X7R characteristic ), Due to the application of DC voltage , Its electrostatic capacity sometimes differs from the nominal value , As can be seen from the figure below , The greater the DC voltage applied , The lower the actual electrostatic capacity .
For temperature compensation capacitors (CH、C0G Characteristics, etc. ) , The main raw material is often induced ceramics , The electrostatic capacity does not change due to the DC voltage characteristics .
C0G The advantage of capacitance like is that the capacitance change caused by temperature is small , The disadvantage is low capacitance , There can't be a lot of capacity .
X5R The advantage of capacitance like is high capacitance , Can have large capacitance , The disadvantage is that the capacity change caused by temperature is large .
Tantalum capacitance
Tantalum capacitors usually look like this , Compared with aluminum electrolytic capacitors , It's still small .
The advantage of tantalum capacitors is ：
 Small size, large capacity
 The leakage current is small
 At higher frequencies ESR low , But no MLCC low
 It has the ability of selfhealing under low current （ Depend on the handle MnO2 become MnO, Fix the defective location ）
The disadvantage is that the ability to withstand voltage and current is relatively weak , Overpressure is easy to explode , Failure of the capacitor may cause open fire .
Capacitor charging and discharging
The time constant must be mentioned when the capacitor is charged or discharged $τ_{e}=RC$, One $τ_{e}$ Represents that the capacitor is charged to the supply voltage 63% Time spent .
The formula of capacitor charging is ：
$V_{c}=V_{0}+(V_{u}−V_{0})∗(1−e_{−RCt})$
among $V_{0}$ It's the starting voltage of the capacitor ,$V_{u}$ It's the voltage at which the capacitor is fully charged , Capacitance from 0V Start filling , namely $V_{0}=0V$, Then the above formula can be simplified as ：
$V_{c}=V_{u}∗(1−e_{−RCt})$
When time is a $τ_{e}$, We can get the following , That is, the capacitor voltage is charged to 63% The origin of , among $e$=2.71828
$V_{c}=V_{u}∗(1−e1 )=0.63V_{u}$
Empathy ,2 individual $τ_{e}$ The time can be charged to the power supply voltage 87%,3 individual $τ_{e}$ It can be charged to the supply voltage 95%.
The formula of capacitance discharge is ：
$V_{c}=V_{u}∗e_{−RCt}$
among $V_{u}$ It's the voltage on the capacitor before discharge .
The function of capacitance
In circuit design, capacitors are often placed in the power supply 、 radio frequency 、 Audio and other places .
 The capacitance of power supply terminal is mainly filter , The larger the volume value , The more stable the output voltage is , The ripple is small .
 The capacitance at the RF end , It is mainly used for power supply and energy storage of radio frequency , In order to prevent instantaneous high current caused by RF power failure , Because of tantalum capacitor ESR Affect efficiency , So for capacitors ESR The requirements are quite high .
 The main function of the series capacitor of audio frequency is to isolate , Larger capacity , The greater the range of audio you pass through , The better the bass effect ,ESR The lower the , The smaller the impact on the output power , The more efficient , But because of the smaller impedance relative to the earphone , So right. ESR Not very demanding .
Capacitance has a wide range of functions , The above is just a few examples .
The actual equivalent model
The ideal capacitor doesn't exist in practice , The actual model of capacitance is a ESR A series of ESL, Another capacitor in series ,ESR Is the equivalent series resistance ,ESL Is the equivalent series inductance ,C It's the ideal capacitor .
So the complex impedance of the above model is ：$Z=ESR+j2πfESL+j2πfc1 =ESR+j(2πfESL−2πfc1 )$
 $2πfESL<<2πfc1 $ when , The capacitor behaves as capacitive ;
 $2πfESL>>2πfc1 $ when , Capacitors behave as perceptual , So there is a saying that capacitance is no longer capacitance at high frequency , It's an inductance , This inductance doesn't mean that a capacitor becomes an inductor , It means that the capacitor has the similar characteristics as the inductance .
 $2πfESL=2πfc1 $ when , The capacitive reactance vector is equal to the inductive reactance vector , The total impedance of the capacitor is the smallest , It shows pure resistance characteristic , At this time f It's called the self resonant frequency of a capacitor .
The self resonance frequency point is the dividing point between capacitance and sensibility , Above resonance point “ A capacitor is no longer a capacitor ”, So decoupling will decrease . The actual capacitor has a certain operating frequency range , Within the operating frequency range , The capacitor has a good decoupling effect .ESL It is the fundamental reason that the decoupling function of the capacitor is weakened after it is higher than the self resonant frequency point .
The figure below shows the frequency characteristics of the actual capacitor .
The actual equivalent model of capacitance can be read from the author's previous article ： Seeing is not necessarily believing ！ resistance 、 Practical equivalent model of capacitance and inductance
Capacitance parameters and selection
Capacitor selection mainly considers the following parameters .
 Capacity ： At room temperature 25℃, The capacity measured under an AC signal of a certain frequency and amplitude .
 Rated voltage ： It refers to the maximum effective value of DC voltage that can be continuously applied to the capacitor at the lowest ambient temperature and rated ambient temperature , Generally, derating is required , Such as derating 80% refer to 6V Capacitance at rated voltage , The value of DC voltage added to it should not exceed 4.8V; But there is one caveat , The rated voltage of the actual capacitor is made with a certain margin , Such as rated voltage 6V, The actual withstand voltage value may reach its rated voltage 1.5 About times .
 Leakage current ： The test conditions are generally at rated voltage , Work 5 Average leakage current per minute . The standard value of DC leakage current is not specified , But the insulation resistance is the specified value , Through the specified value of insulation resistance and rated voltage of the product , Using the formula I=V/R Calculate the leakage current , That is, the greater the insulation resistance of the capacitor , The smaller the leakage current .
 Insulation resistance ： At room temperature , Charge the capacitor at rated voltage 1 minute /2 minute , Divide the voltage value by 1 minute /2 The average leakage current per minute is used to obtain the insulation resistance value .
 ESR： It refers to the equivalent series resistance of a capacitor , Its parameters can be SPEC View in ,ESR The value of affects the power ripple and PDN Simulation ,MLCC Of ESR It's usually very small ,mΩ Level , Tantalum capacitors and aluminum electrolytic capacitors are generally Ω Level .
Common capacitor brands
Common capacitor brands such as Murata MURATA、 sis KEMET、AVX、TDK、 Weishi VISHAY、 Yuyang 、 National giant Yageo etc. .
This is the end of today's article , I hope it helps you , There should be another article about capacitors , I'll see you in the next issue .
版权声明
本文为[Remember sincerity]所创，转载请带上原文链接，感谢
https://chowdera.com/2020/12/202012042254512124.html
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