First, the basic principle of capacitance
Capacitors, together with inductors and resistors, are the three basic passive devices in electronics. The function of a capacitor is to store electrical energy in the form of electric field energy.
Taking parallel plate capacitor as an example, the basic principle of capacitor is briefly introduced.
As shown in the figure above, a DC voltage is applied to two metal plates that are close and parallel to each other (the dielectric between the plates); After stabilization, the metal plate connected to the positive electrode will show a certain amount of positive charge, while the metal plate connected to the negative electrode will show an equal amount of negative charge. In this way, an electrostatic field is formed between the two metal plates, so the capacitor stores electrical energy in the form of electric field energy, and the amount of charge stored is Q.
The amount of charge Q stored by the capacitor is related to the voltage U and its own properties (that is, the capacitance value C), that is, Q=U*C. According to theoretical derivation, the capacitance formula of parallel plate capacitor is as follows:
Inside the ideal capacitor is the Dielectric (Dielectric), there is no free charge, it is impossible to produce charge movement that is, current, so how does the ideal capacitor communicate?
communicate
The voltage can create an electric field inside the capacitor, and the AC voltage creates an alternating electric field. According to the total current law in Maxwell's equations:
That is, an electric current or a changing electric field can produce a magnetic field, and Maxwell defines ε(∂E/∂t) as a displacement current, which is an equivalent current and represents a change in the electric field. (Where the current represents the current density, i.eJ)
Let the AC voltage change sinusoidal, that is:
The actual displacement current equals the current density multiplied by the area:
Therefore, the capacitive reactance of the capacitor is 1/ωC, and when the frequency is very high, the capacitive reactance will be very small, that is, the high frequency.
The following figure is the change of electromagnetic field inside the parallel plate capacitor simulated by ANSYS HFSS.
Cross-sectional electric field change (GIF, seems to click to see)
Profile magnetic field change (GIF, seems to click to see)
That is to say, when the capacitor communicates, the internal electric and magnetic fields are converted to each other.
Separate direct current
The DC voltage does not change with time, the displacement current ε(∂E/∂t) is 0, and the DC component cannot pass through.
Actual capacitance equivalent model
The characteristics of actual capacitors are not ideal, and there are some parasitic effects. Therefore, it is necessary to use a more complex model to represent the actual capacitance, and the commonly used equivalent model is as follows:
Because the medium is not absolutely insulated, there is a certain conductive capacity; Therefore, any capacitor has a leakage current, which is represented by the equivalent resistance Rleak;
The conductor and electrode of the capacitor have certain resistivity, and the dielectric has certain dielectric loss. These losses are uniformly expressed as equivalent series resistance ESR;
There is a certain inductance in the conductor of the capacitor, which has a great influence at high frequency, and is represented by the equivalent series inductance ESL;
In addition, any medium has a certain hysteresis phenomenon, that is, after the capacitor is discharged quickly, the voltage is suddenly disconnected, and the capacitor will recover part of the charge, which is expressed by a series RC circuit.
Most of the time, the focus is on the ESR and ESL of capacitors.
Quality Factor
As with inductors, you can define the quality factor of the capacitor, that is, the Q value, that is, the ratio of the stored power to the lost power of the capacitor:
Qc = (1 / C) omega/ESR
Q value is an important parameter for high frequency capacitance.
Self-Resonance Frequency
Due to the existence of ESL, a resonant circuit is formed with C, and its resonant frequency is the self-resonant frequency of the capacitor. Before the self-resonant frequency, the impedance of the capacitor decreases with the increase of frequency. After the self-resonant frequency, the impedance of the capacitor decreases with the increase of the frequency, so it is inductive. As shown in the picture below:
Image from Taiyo Yuden's EMK042BJ332MC-W specification
Second, The Process And Structure Of Capacitors
According to the capacitance formula, in addition to the size of the capacitor, the size of the capacitance is related to the Permittivity of the dielectric. The performance of the dielectric affects the performance of the capacitor, and different media are suitable for different manufacturing processes.
The manufacturing process of capacitors can be mainly divided into three categories:
Film Capacitor
Electrolytic Capacitor (Electrolytic Capacitor)
Ceramic Capacitor
2.1 Film Capacitor
Film Capacitor is usually translated as Thin Film capacitor in China, but it is different from the thin film process. In order to distinguish, personally think the direct translation as membrane capacitance is good point.
The film capacitor is made by winding two plastic films with metal electrodes into a cylindrical shape, and finally packaging; Because its medium is usually a plastic material, it is also known as a plastic film capacitor; Its internal structure is roughly as shown in the figure below:
Thin film capacitors can be divided into two categories according to the production process of their electrodes:
Metal Foil Film capacitor (Film/Foil)
Metal foil film capacitor, directly on the plastic film to add a thin layer of metal foil, usually aluminum foil, as the electrode; This process is relatively simple, the electrode is convenient to draw out, and can be applied to high current situations.
Metallized Film
Metallized film capacitor, through the Vacuum deposition process directly on the surface of the plastic film to form a very thin metal surface, as an electrode; Because the electrode thickness is very thin, it can be wound to make a larger capacity capacitor; However, due to the thin electrode thickness, it is only suitable for small current situations.
The metallized film capacitor has the function of self-repair, that is, if there is a breakdown damage point inside the capacitor, there will be an avalanche effect at the damage place, the gasification metal will form a gasification collection surface at the damage place, the short circuit will disappear, and the damage point will be repaired; Therefore, the capacitor reliability of the metallized film is very high, and there is no short-circuit failure;
Film capacitors have two winding methods: inductive winding before winding, the lead is already connected to the internal electrode; After winding, the non-inductive winding method will use gold-plating and other processes to connect the internal electrodes of the two end faces into a surface, which can obtain smaller ESL and should have higher high-frequency performance; In addition, there is a laminated non-inductive capacitor, similar to MLCC structure, better performance, easy to make SMD package.
The earliest film capacitor dielectric material is paper dipped in oil or paraffin, the British D 'Fitzgerald invented in 1876; The operating voltage is very high. Now multi-purpose plastic materials, that is, polymer, according to the different medium materials, there are mainly the following:
The most widely used film capacitor is the polyester film capacitor, which is relatively cheap and can be made smaller due to its high dielectric constant. The second is the polypropylene film capacitor. Other materials are polytetrafluoroethylene, polystyrene, polycarbonate and so on.
The characteristics of thin film capacitor is that it can achieve large capacity and high voltage; However, due to process reasons, its size is difficult to make small, and it is usually used in strong electric circuits, such as the power electronics industry; It basically looks like this:
Screenshot from High Power Capacitors For Power Electronics - AVX
2.2 Electrolytic Capacitor
Electrolytic capacitors use metal as an Anode, and form a layer of metal oxide film on the surface as a medium; A wet or solid electrolyte and metal then act as a Cathode. Electrolytic capacitors are mostly polar, if the cathode side of the metal, there is also a layer of oxide film, is a non-polar electrolytic capacitor.
Depending on the metal used, there are currently only three types of electrolytic capacitors:
Aluminum electrolytic capacitors
Aluminum electrolytic capacitors should be the most widely used electrolytic capacitors, the cheapest, its basic structure is shown in the following figure:
The production process of aluminum electrolytic capacitor is roughly as follows:
First of all, the aluminum foil will form a very rough surface through electroetching, which increases the surface area of the electrode and increases the capacitance;
Then an oxide layer is formed by chemical anodizing as a medium;
Then, a layer of electrolytic paper is added between the anode aluminum foil and the cathode aluminum foil as isolation, pressing and winding;
Finally, add the electrolyte, the electrolytic paper will absorb the electrolyte, packaging molding.
Wet aluminum electrolytic capacitors using electrolytes are the most widely used; The advantages are large capacity, high rated voltage and cheap; Disadvantages are also obvious, that is, short life, poor temperature characteristics, and large ESR and ESL. For hardware development, it is necessary to avoid over-design, and in the case of meeting performance requirements, cheap is the biggest advantage.
The following figure is Kemet's aluminum electrolytic capacitor products, which can roughly see the characteristics of aluminum electrolytic capacitors.
Original image screenshot from KEMET website
Aluminum electrolytic capacitors also use solid materials such as manganese dioxide and conductive polymer as electrolytes; The structure of the polymer aluminum electrolytic capacitor is roughly as shown in the figure below:
Original image from Polymer Aluminum Electrolytic Capacitors-Murata
The polymer aluminum electrolytic capacitor has smaller ESR, more stable capacity and better transient response. Because it is solid, the impact vibration resistance is better than the wet type; Smaller SMD packages can be made. Of course, wet aluminum electrolytic capacitors can also be SMD packages:
Tantalum electrolytic capacitors
The most widely used electrolytic capacitor of tantalum should be the use of manganese dioxide as a solid electrolyte, the main length is this:
Image from Solid Tantalum MnO2 Capacitors
The internal structure of solid tantalum electrolytic capacitor is roughly as shown in the figure below:
Original image from Vishay technical documentation
The ratio of tantalum capacitor to aluminum electrolytic capacitor is that the dielectric constant of tantalum oxide (tantalum pentoxide) is much higher than that of aluminum oxide (aluminum oxide), so that the same volume of tantalum capacitor capacity is larger than that of aluminum electrolytic capacitor. Tantalum capacitor has a long life and more stable electrical performance.
Tantalum capacitors also use Conductive Polymer as the electrolyte, the structure of which is similar to that of the manganese dioxide tantalum capacitor in the figure above, that is, the manganese dioxide is replaced by conductive polymer; The conductive polymer has a higher conductivity than manganese dioxide, so the ESR will be lower.
In addition, there are wet tantalum capacitors, which are characterized by super-large capacity, high voltage resistance and low DC leakage current, mainly used in military and aerospace fields.
Niobium electrolytic capacitors
Niobium electrolytic capacitor is similar to tantalum electrolytic capacitor, that is, niobium and its oxide instead of tantalum; The dielectric constant of niobium oxide (niobium pentoxide) is higher than that of tantalum oxide (tantalum pentoxide). The performance of niobium capacitor is more stable and the reliability is higher.
AVX has a niobium capacitor series, the appearance of manganese dioxide tantalum capacitors is yellow, while the appearance of niobium capacitors is orange-red:
Electrolytic capacitor comparison table, data from Wikipedia, for reference only.
2.3 Ceramic Capacitor
Ceramic capacitors are based on ceramic materials as dielectric materials, there are many kinds of ceramic materials, dielectric constant, stability are different, suitable for different occasions.
Ceramic capacitors, there are mainly the following types:
Ceramic Disc Capacitor
The main advantage of the ceramic capacitor is that it can withstand high voltage, which is usually used as a safety capacitor and can withstand 250V AC voltage. Its appearance and structure are shown below:
The original image is taken from the two extended readings in this section
Multi-layer Ceramic Capacitor
Multilayer ceramic capacitors, also known as MLCC, Chip multilayer ceramic capacitors are currently the most used type of capacitors in the world, and their standardized packaging and small size are suitable for automated high-density patch production.
The author, that is, I designed the motherboard myself, took the photos myself, and added the artistic effect; Most of the pictures and content without reference and source are drawn or made by me, and the rest may be negligent and forgotten to add; Many of the pictures quoted by the standard are reprocessed by me, such as translation or several pictures put together, etc., and the tools are very primitive EXCEL+ screenshots.
The internal structure of the multi-layer ceramic capacitor is shown as follows:
Original image from SMD MLCC for High Power Applications - KEMET
The production process of multi-layer ceramic capacitor is shown as follows:
Original image from Capacitors, Part 2 "Ceramic Capacitors
Because the multi-layer ceramics need to be sintered porcelain to form an integrated structure, the multi-layer ceramic capacitors packaged with Lead wires are also called Monolithic capacitors.
Multilayer ceramic process and Thin Film process are also introduced in talking about inductors. Thin Film technology is more advanced in terms of performance or process control, and can accurately control the electrical and physical properties of the device. Therefore, the Thin Film capacitance performance is relatively good, the minimum tolerance can be 0.05pF, and the tolerance can be 0.01pF; Much better than the usual MLCC, like Murata's GJM series, the minimum tolerance is 0.1pF, and the tolerance is usually 0.05pF; Therefore, Thin Film capacitors can be used in the more demanding RF field, AVX has the Accu-P® series.
Classification of ceramic media
According to EIA-198-1F-2002, ceramic media are mainly divided into four categories:
Class I: Ceramic media with temperature compensation characteristics, its dielectric constant is mostly low, not more than 200. It is usually a Paraelectric medium, and the dielectric constant is relatively stable and changes little under temperature, frequency and bias voltage. The loss is also very low, and the dissipation factor is less than 0.01.
Screenshot from Materials Development for Commercial Multilayer Ceramic Capacitors, Page26
The most stable and most widely used is the C0G capacitor, that is, NP0. NP0 is the code specified in the IEC/EN 60384-1 standard, that is, Negative Positive Zero, that is, N and P are used to indicate positive and negative deviations.
Due to the low dielectric constant, the capacity of the C0G capacitor is small, and the maximum can be 0.1uF, and the 0402 package is usually only 1000pF.
Class II, III: Among them, the temperature characteristic A-S belongs to Class II, and the dielectric constant is about several thousand. The temperature characteristic T-V belongs to Class III, and the dielectric constant can be up to 20000, which can be seen that the performance of Class III is more unstable. According to the IEC classification, both Class II and III belong to the second class, high dielectric constant media. Like X5R and X7R are Class II capacitors, which are used more in power decoupling, while Y5V belongs to Class III capacitors, whose performance is not stable, and I personally feel that there are not many applications now.
Screenshot from Materials Development for Commercial Multilayer Ceramic Capacitors, Page103
Because the capacitance value of Class II and III capacitors can be up to several hundred uF, but because of the high dielectric constant media, mostly Ferroelectric media (Ferroelectric), the temperature stability is poor. In addition, the dielectric constant of ferroelectric media decreases under DC bias voltage.
In the article about inductance, it is introduced that ferromagnetic medium has hysteresis phenomenon, when the internal magnetic field exceeds a certain value, magnetic saturation phenomenon will occur, resulting in the decrease of permeability. Similarly, for ferroelectric media, there is electric hysteresis phenomenon, when the internal electric field exceeds a certain value, electric saturation phenomenon will occur, resulting in a decrease in dielectric constant.
Therefore, when the DC bias voltage of Class II and III capacitors exceeds a certain value, the capacitance will drop significantly, as shown in the figure below:
Image credit: GRM188R60J226MEA0-Murata
Class IV:The production process is different from the usual ceramic materials, the internal ceramic particles are a thin layer of oxide on the outside, and the core is a conductor. This type of capacitor has a large capacity but a small breakdown voltage. Due to the unstable performance and high loss of such capacitors, they have been basically eliminated.
Capacitor type summary table
Original image from Wikipedia
There is also a class of super capacitors, that is, the capacity is particularly large, can replace the battery as a power supply device, and can also be used with the battery. Supercapacitors charge fast, can be fully charged and discharged, and can be charged to any desired voltage, as long as it does not exceed the rated voltage. Now there are more applications, many cities in China have super capacitor electric buses; There are also applications in some electronic products, such as some dashcams, which can last for several days.
3. Application And Selection Of Capacitors
Device selection, in fact, is to extract relevant information from the device specification book to determine whether it meets the requirements of product design and application.
3.1 Overview
As an energy storage component, a capacitor can store energy. The capacitor may also be charged after the external power supply is disconnected. Therefore, safety tips are necessary. Some electronic equipment will be attached to the inside of a high voltage danger, when I was a child to dismantle the black and white TV at home, opened to see the picture tube attached to a high voltage danger, then there is a question, no power will also have high voltage? After the work, several power adapters have been removed, and the aftertaste of electricity is endless...
To return to the subject, capacitive energy storage can be used in the following applications:
Stored energy can be used as a power source, such as a supercapacitor;
Storage of data, very wide applications. Dynamic volatile memory (DRAM) is the use of integrated capacitor arrays to store data, the capacitor is fully charged is 1, discharged is 0. The amount of memory used in various mobile phones, computers and servers is very large, so the memory industry can be used as a weather vane for the information industry.
In addition, capacitors can be used as:
Timing: capacitor charge and discharge take time, can be used as a timer; You can also do delay circuit, the most common is power-on delay reset; Some timing chips, such as the NE556, can generate triangular waves.
Resonant source: Together with the inductor, the LC resonant circuit is formed to produce a fixed frequency signal.
Using the characteristics of the capacitor through high frequency, low frequency, and DC isolation, the capacitor can also be used as:
Power decoupling
Power decoupling should be the most widespread application of capacitors, and a large number of capacitors are placed around and on the back of various cpus, SoCs, and ASics to maintain the stability of the supply voltage.
First, in the DCDC circuit, it is necessary to select the appropriate input capacitance and output capacitance to reduce voltage ripple. The relevant parameters need to be calculated.
In addition, when IC is working, the working current required at different times is not the same, therefore, a large number of decoupling capacitors are also required to ensure that the working voltage is stable.
Coupling straightening
When designing a circuit, in some cases, you only want to transmit AC signals and do not want to transmit DC signals, so you can use series capacitors to couple signals.
For example, in order to prevent the mutual influence of DC bias, the calculation of static operating points is complicated, and capacitive coupling is usually used between stages, so that each stage static operating point can be analyzed independently.
High-speed serial signals, such as PCIE and SATA, usually also use capacitors for AC coupling.
Bypass filtering
Bypass, as the name suggests, is to bring unwanted AC signals into the ground. Filtering actually means the same thing. In microwave RF circuits, the design of various filters requires the use of capacitors. In addition, like the EMC design, for the LED light at the interface, a filter capacitor will be added to the signal line, which can improve the reliability of ESD testing.
3.2 Aluminum Electrolytic Capacitor
3.2.1 Aluminum electrolytic Capacitor (wet)
Aluminum electrolytic capacitors (wet type) Whether plug-in or patch package, the height is relatively high, and the ESR is high, not suitable for placing near the IC to do power decoupling, usually for the input and output capacitors of the power circuit.
Original drawing from KEMET specification
tolerance
Obtain the capacitance value tolerance from the specification, usually the tolerance of aluminum electrolytic capacitors is ±20%. When calculating the maximum and minimum capacity values, the parameters should meet the design requirements.
Rated voltage
Aluminum electrolytic capacitors are usually only suitable for DC applications, and the designed operating voltage should be at least 80% lower than the rated voltage. For a circuit with surge protection, the rated surge voltage is higher than the residual voltage of the protective device (usually TVS).
For example, for some POE-powered devices, according to the 802.3at standard, the operating voltage can reach up to 57V, then the TVS clamp voltage selected has more than 90 V, then at least select an aluminum electrolytic capacitor with a rated voltage of 100V. At this time, only aluminum electrolytic capacitors can simultaneously meet the requirements of large capacity.
Original image from Littelfuse TVS specification
Dissipation factor
When designing a DCDC circuit, the ESR of the output capacitor affects the output voltage ripple, so it is necessary to know the ESR of the aluminum electrolytic capacitor, but most of the specifications of the aluminum electrolytic capacitor only give the dissipation factor tanδ. ESR can be calculated according to the following formula:
ESR = tanδ/(2πfC)
For example, at 120Hz, the tanδ is 16% and the C is 220uF, so the ESR is about 965 Mω. It can be seen that the ESR of the aluminum electrolytic capacitor is very large, which will lead to a large output voltage ripple. Therefore, when using aluminum electrolytic capacitors, it is necessary to use chip ceramic capacitors, placed close to the DCDC chip.
With the increase of switching frequency and temperature, ESR will decrease.
Rated ripple current
The ripple current of the capacitor needs to meet the RMS current requirements of the input and output capacitors of the DCDC design. The rated ripple current of the aluminum electrolytic capacitor needs to be corrected according to the switching frequency.
Life span
The life of aluminum electrolytic capacitors is relatively short, and the selection needs attention. The life is directly related to the operating temperature, and the specification usually gives the life of the product at the highest temperature, such as 105 ° C, the life is 2000 hours.
According to the rule of experience, every 10℃ decrease in the working temperature, the life is multiplied by 2. If the product is designed for a service life of 3 years, that is 26,280 hours. 10*log2(26280/2000)=37.3 ° C, then the design operating temperature can not exceed 65 ° C.
3.2.2 Polymer aluminum electrolytic capacitor
For high-power devices such as Intel's CPU, a chip has a power consumption of more than 80 watts, and a nuclear power stream of tens to hundreds of amPs, while the main frequency is very high and the high-frequency components are many. At this time, the requirements for decoupling capacitors are high:
The capacitance value should be large to meet the requirements of large current;
The rated RMS current should be large to meet the requirements of large current;
The ESR should be small to meet the requirements of high frequency decoupling;
The capacity stability is better.
Surface mount, the height can not be too high, because it is usually placed on the BOTTOM layer of the back of the CPU to achieve the best decoupling effect.
At this time, the choice of polymer aluminum electrolytic capacitor is most suitable.
In addition, for audio circuits, it is usually necessary to use coupling and decoupling capacitors, because the frequency of audio is very low, so you need to use large capacitors, at this time polymer aluminum electrolytic capacitors are also suitable.
3.3 Tantalum Capacitor
According to the sources of the previous relevant information, it can be found that the main manufacturers of tantalum capacitors are Kemet, AVX, and Vishay.
Tantalum is a relatively rare metal, so tantalum capacitors will be a little more expensive than other types of capacitors. However, the performance is better than the aluminum electrolytic capacitor, the ESR is smaller, the loss is smaller, the decoupling effect is better, and the leakage current is small. The following is the parameter table of a solid tantalum capacitor by Kemet:
Screenshot from Kemet specification
Rated voltage
The working voltage of solid tantalum capacitor needs derating design. Under normal conditions, the operating voltage should be less than 50% of the rated voltage; In high temperature environment or low load impedance, the operating voltage should be less than 30% of the rated voltage. The specific reduction requirements should strictly comply with the specifications.
In addition, it is also necessary to pay attention to the reverse voltage of the tantalum capacitor, which may cause the tantalum capacitor to withstand the reverse voltage, resulting in the failure of the tantalum capacitor.
The main failure mode of solid tantalum capacitor is short circuit failure, which will directly lead to the risk of circuit failure and even fire. Therefore, extra attention needs to be paid to reliability design to reduce failure rate.
For products that will cause major accidents if they fail, it is recommended not to use solid tantalum capacitors.
Rated ripple current
The ripple current flowing through the tantalum capacitor will cause the temperature rise of the tantalum capacitor due to the presence of ESR, and the ambient temperature should not exceed the rated temperature of the tantalum capacitor and the relevant derating design.
3.4 Sheet Multilayer Ceramic Capacitor
Chip multilayer ceramic capacitors should be the largest shipments of capacitors, and there are many manufacturers, like the three major Japanese TDK, muRata, Taiyo Yuden, and the American system like KEMET and AVX(which has been acquired by Japan's Kyocera).
The three major Japanese systems do better is to have the corresponding selection software, inductors, capacitors and other all series of products and related parameter curves, very complete, have to recommend again:
3.4.1 Class I Capacitor
The most widely used Class I capacitor is C0G capacitor, which has stable performance and is suitable for high-frequency circuits such as resonance, matching and filtering.
The capacity of the C0G capacitor is very stable, and basically does not change with external conditions (except frequency). The following diagram shows the DC, AC and temperature characteristics of a Murata 1000pF capacitor.
Image from GRM1555C1H102JA01-Murata
Therefore, it is usually only necessary to focus on the frequency characteristics of the C0G capacitor. Below is a comparison of the frequency characteristics of three Murata 10pF capacitors in the same package (0402inch) with the same tolerance (5%).
Image from Simsurf-Web-Murata
Among them, GRM is a common series, GJM is a high Q value series, GQM is a high frequency series, it can be seen that the high frequency performance of the GQM series is better, the self-resonant frequency and Q value is higher, some high frequency performance requirements, you can choose 1% of the product. The GRM series is cheaper and more versatile, such as EMC filtering.
3.4.2 Class II and Class III capacitors
Class II and Class III capacitors are high dielectric constant media, unstable performance, large capacitance range, usually used as power decoupling or signal bypass.
Take a Murata 22uF, 6.3V, X5R capacitor as an example, the relevant characteristic curve:
Image from GRM188R60J226MEA0-Murata
tolerance
Class II and Class III capacitors have a wide range of tolerance variations with temperature, DC bias, and AC bias. Especially when used as a power supply decoupling, the capacitor has a certain DC bias, and the capacitance is much smaller than the nominal value, so pay attention to whether the actual capacity meets the design requirements.
Ripple current
As input and output capacitors of DCDC, there will be a certain ripple current, and the presence of ESR will lead to a certain temperature rise. Coupled with the ambient temperature, the rated temperature of the capacitor cannot be exceeded, for example, the maximum temperature of the X5R capacitor is 85 ° C.
Generally, because the multi-layer ceramic capacitor ESR is small, it can withstand larger ripple current.
Self-resonant frequency
Capacitors have a self-resonant frequency due to the presence of ESL. Large capacity capacitor, self-resonant frequency is low, only 1-2MHz. Therefore, in order to improve the high frequency effect of the power supply, a large number of decoupling capacitors with small capacitance values are necessary. In addition, for DCDC chips with high switching frequency, pay attention to the self-resonant frequency of the input and output capacitors.
ESR
To design a DCDC circuit, it is necessary to know the ESR of the output capacitance to calculate the output voltage ripple. The ESR of multilayer ceramic capacitors is usually low, about a few to tens of milliohm.
3.5 Safety Capacitor
For our home electronic equipment, the final is 220V AC mains power supply. In order to reduce interference to the power grid, the power adapter will add various filtering capacitors through relevant EMC tests. Below is a simple schematic of the circuit:
The capacitor between L and N is called the X capacitor, and the capacitor between L, N and PE or GND is called the Y capacitor. Because 220V AC is dangerous and threatens personal safety, electronic products need to meet the relevant safety standards, such as GB4943 and UL60950 test requirements. Therefore, X capacitors and Y capacitors are directly related to these tests, so they are also called safety capacitors.
Take the electrical strength test as an example. According to the standard, the L and N sides are primary circuits, and the basic insulation between them and PE or GND is required. Therefore, it is necessary to add AC 1.5kV or DC 2.12kV withstand voltage test between L or N and GND for nearly 1 minute, during which the relevant leakage current should not exceed the standard value. Therefore, the safety capacitor has quite high voltage requirements, and the DC leakage current cannot be too large.
In addition, the commonly used RJ45 network port, in order to reduce EMI, commonly used Bob-Smith circuit, as shown in the following figure:
It can be seen that the voltage withstand of the capacitor is more than 2kV, because the network port usually has a transformer, 220V AC L and N to the network cable have two transformers isolated, is double insulation, and the electrical strength test should also be carried out between L and N to the network cable. Double insulation, usually required to pass the AC 3kV or DC 4.24kV test.
Because safety capacitors have high voltage requirements, porcelain capacitors or small film capacitors are usually used.
In addition, the device selection also has two main requirements: and structure to confirm the length, width and height of the device; For the plug-in package device is not too much, is it possible to use all the surface attachment device, so as to avoid the wave soldering process.