Summary of capacitors, inductors and magnetic beads in hardware design

Common capacitors are:

Aluminum electrolytic capacitor: polarity, large capacity, can withstand large pulsating current, but has large capacity error and leakage current, suitable for low-frequency bypass, signal coupling and power supply filtering.

Gallbladder electrolytic capacitor: It has the characteristics of ordinary electrolytic capacitors, with extremely small leakage current, long life, small capacity error, and small size, making it suitable for small devices.

Film capacitors: They are non-polar capacitors used in differential filters, integration, oscillation and timing circuits.

Ceramic capacitors: Non-polar capacitors are suitable for high-frequency bypass.

Ceramic capacitor: It is a non-polar capacitor, which includes high-frequency ceramic capacitor and low-frequency ceramic capacitor. It is suitable for high- and low-frequency circuits, but not suitable for pulse circuits, otherwise it is easy to break down.

In addition, when determining the polarity of the electrolytic capacitor, for a plug-in electrolytic capacitor, the end with a white mark or a shorter lead is the negative pole; if it is a chip electrolytic capacitor, the end with a horizontal bar mark is the positive pole.

Diodes and transistors:

For example, 3AX82_81,

The naming method of materials: A - N type germanium material, B - P type germanium material, C - N type silicon material, D - P type silicon material.

The types are named as follows: P—ordinary tube, W—Zener diode.

In the triode, the materials are named as follows: A-PNP type germanium material B-NPN type germanium material C-PNP type silicon material D-NPN type silicon material.

The types are named as follows: Z - rectifier tube, X - low-frequency low-power tube, U - phototube, K - switching tube, CS - field effect tube.

Magnetic bead inductor:

In order to filter out the noise interference of the power supply circuit to the system, an inductor or magnetic bead is often added to the power supply output to filter out the noise brought by the power supply circuit. The inductor filtering is a reflective filtering, which attenuates signals of various frequencies, while the magnetic bead is an absorptive filtering, which only has a large attenuation on the 1KHz signal and a small attenuation on other signals. The heat dissipation of magnetic beads sometimes needs to be considered, otherwise it will affect their magnetic conductivity. Nominal value: Because the unit of the magnetic bead is nominal according to the impedance it generates at a certain frequency, the unit of impedance is also ohm. Generally, 100MHz is used as the standard. For example, 2012B601 means that the impedance of the magnetic bead is 600 ohms at 100MHz. Rated current: Rated current refers to the current that can ensure the normal operation of the circuit. The difference between inductors and magnetic beads: Coils with more than one turn are usually called inductor coils, and coils with less than one turn (conductors passing through the magnetic ring) are usually called magnetic beads; inductors are energy storage components, while magnetic beads are energy conversion (consumption) devices; inductors are mostly used in power supply filter circuits, and magnetic beads are mostly used in signal circuits for EMC countermeasures; magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors are used in this regard to focus on suppressing conductive interference. Both can be used to deal with EMC and EMI problems; inductors are generally used for circuit matching and signal quality control. Magnetic beads are used where analog ground and digital ground are combined. Magnetic beads have high resistivity and magnetic permeability. They are equivalent to resistors and inductors in series, but the resistance and inductance values ​​vary with frequency. It has better high-frequency filtering characteristics than ordinary inductors, and is resistive at high frequencies, so it can maintain a high impedance in a fairly wide frequency range, thereby improving the FM filtering effect. Inductors can be used as power supply filters. The circuit symbol of the magnetic bead is the inductor, but the model number shows that the magnetic bead is used. In terms of circuit function, the magnetic bead and the inductor have the same principle, but the frequency characteristics are different.

The magnetic bead is composed of an oxygen magnet, and the inductor is composed of a magnetic core and a coil. The magnetic bead converts the AC signal into heat energy, and the inductor stores the AC and releases it slowly. The magnetic bead has a greater blocking effect on high-frequency signals. The general specifications are 100 ohms/100mMHZ. Its resistance is much smaller than that of the inductor at low frequencies. Ferrite beads are a kind of anti-interference component that is currently developing rapidly. They are cheap, easy to use, and have a significant effect in filtering high-frequency noise. In the circuit, as long as the wire passes through it (I use those that look like ordinary resistors, the wires have been passed through and glued, and there are also surface mount forms, but they are rarely sold). When the current passes through the wire, the ferrite has almost no impedance to the low-frequency current, but it will have a greater attenuation effect on the higher-frequency current. The high-frequency current is dissipated in the form of heat, and its equivalent circuit is an inductor and a resistor in series, and the values ​​of the two components are proportional to the length of the magnetic bead. There are many types of magnetic beads, and the manufacturer should provide technical indicators, especially the curve of the relationship between the impedance and frequency of the magnetic beads. Some magnetic beads have multiple holes. Passing a wire through them can increase the impedance of the component (the square of the number of times the wire passes through the magnetic bead). However, the increased noise suppression capability at high frequencies may not be as much as expected, and it would be better to connect more magnetic beads in series. Ferrite is a magnetic material that will produce magnetic saturation and a sharp drop in magnetic permeability due to excessive current passing through it. High current filtering should use specially designed magnetic beads, and attention should also be paid to their heat dissipation measures.

Ferrite beads can not only be used to filter high-frequency noise in power supply circuits (can be used for DC and AC output), but can also be widely used in other circuits, and their volume can be made very small. Especially in digital circuits, since pulse signals contain high-frequency high-order harmonics, which are also the main source of high-frequency radiation in circuits, magnetic beads can play a role in this occasion.

Ferrite beads are also widely used in noise filtering of signal cables.

Note: For diodes (DIODE), DIODExx, the number xx indicates power. The larger the value, the greater the power. It also indicates the distance between two solder joints. For non-polar capacitors RADxx and polar capacitors (RB.2/.4～RB.5/1.0), resistors (AXIAL0.3～AXIAL1.0) 300mil, 1000mil, and variable resistors (VR1～VR5), the number xx indicates the distance between two solder joints.

Clock pins (active crystal oscillator): Pin 1 - floating, Pin 2 - grounded, Pin 3 - output, Pin 4 - power supply

CPLD_JTAG1 pins: 1-TCK, 2-GND, 3-TDO, 4-VDD, 5-TMS, 6/7/8 empty, 9-TDI, 10-GND

1) Aluminum capacitors are made of aluminum foil that is grooved and oxidized, then rolled with an insulating layer, and then immersed in electrolyte. The principle is chemical. The charging and discharging of capacitors depends on chemical reactions. The response speed of capacitors to signals is limited by the movement speed of charged ions in the electrolyte. They are generally used in filtering situations with low frequencies (below 1M). ESR is mainly the sum of aluminum foil resistance and electrolyte equivalent resistance, and the value is relatively large. The electrolyte of aluminum capacitors will gradually evaporate, resulting in a decrease in capacitance or even failure, and the evaporation rate will accelerate as the temperature rises. The life of electrolytic capacitors will be halved for every 10-degree increase in temperature. If a capacitor can be used for 10,000 hours at room temperature of 27 degrees, it can only be used for 1,250 hours at 57 degrees. Therefore, try not to place aluminum electrolytic capacitors too close to heat sources.

2) Ceramic capacitors store electricity through physical reactions, so they have a very high response speed and can be applied to G-level applications. However, ceramic capacitors also show great differences due to different dielectrics. The best performance is the capacitor made of C0G material, which has a small temperature coefficient, but the material has a small dielectric constant, so the capacitance cannot be too large. The worst performance is the Z5U/Y5V material, which has a large dielectric constant, so the capacitance can be as high as tens of microfarads. However, this material is seriously affected by temperature and DC bias (DC voltage will cause the material to polarize and reduce the capacitance).

Generally, large-capacity capacitors are mainly used for filtering. The speed requirement is not very fast, but the capacitance requirement is relatively high. Aluminum electrolytic capacitors are generally used. When the surge current is small, it will be better to use tantalum capacitors instead of aluminum electrolytic capacitors. From the above example, we can know that as a decoupling capacitor, it must have a very fast response speed to achieve the effect. If the local circuit A in the figure refers to a chip, then the decoupling capacitor must be a ceramic capacitor, and the capacitor should be as close to the power pin of the chip as possible. If "local circuit A" refers to a functional module, ceramic capacitors can be used. If the capacity is not enough, tantalum capacitors or aluminum electrolytic capacitors can also be used (the premise is that each chip in the functional module has a decoupling capacitor-ceramic capacitor).

The capacity of the filter capacitor can often be calculated from the data sheet of the switching power supply chip. If the filter circuit uses electrolytic capacitors, tantalum capacitors and ceramic capacitors at the same time, put the electrolytic capacitors as close to the switching power supply as possible to protect the tantalum capacitors. Put the ceramic capacitors behind the tantalum capacitors. This will achieve the best filtering effect.

Decoupling capacitors need to meet two requirements, one is the capacity requirement, the other is the ESR requirement. That is to say, the decoupling effect of a 0.1uF capacitor may not be as good as that of two 0.01uF capacitors. Moreover, 0.01uF capacitors have lower impedance in higher frequency bands. In these frequency bands, if a 0.01uF capacitor can meet the capacity requirement, it will have a better decoupling effect than a 0.1uF capacitor.

Many high-speed chip design manuals with many pins will give the requirements for decoupling capacitors in power supply design. For example, a BGA package with more than 500 pins requires a 3.3V power supply with at least 30 ceramic capacitors and several large capacitors, with a total capacity of more than 200uF...

Each input has 10nF and 100nF noise filters, and in order to stabilize the voltage drop, a 10uF large capacitor is connected. Generally speaking, small capacitors need to be close to the chip, and one for each pin. Large capacitors can be placed farther away.

For the power output part, in addition to general principles, it is necessary to consider the possibility that the peak current of the device is large and the level may be pulled down. Therefore, a large capacitor is required, generally more than 10uF. The typical decoupling capacitor value in digital circuits is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz, that is to say, it has a good decoupling effect for noise below 10MHz, and has almost no effect on noise above 40MHz. The 1μF and 10μF capacitors have a parallel resonance frequency above 20MHz, and the effect of removing high-frequency noise is better. For every 10 integrated circuits or so, add a charging and discharging capacitor, or 1 energy storage capacitor, which can be selected to be around 10μF.

14.1. General Configuration Principles of Decoupling Capacitors

1. Connect a 10~100uf electrolytic capacitor across the power input terminal. If possible, it is better to connect a capacitor of 100uf or more.

2. In principle, each integrated circuit chip should be equipped with a 0.01pf ceramic capacitor. If the printed circuit board space is insufficient, a 1 to 10pf capacitor can be arranged for every 4 to 8 chips.

3. For devices with weak noise immunity and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and ground line of the chip.

4. The capacitor leads should not be too long, especially high-frequency bypass capacitors should not have leads. In addition, the following two points should be noted:

a. When there are contactors, relays, buttons and other components on the printed circuit board, they will generate large spark discharges when they are operated. The RC circuit shown in the attached figure must be used to absorb the discharge current. Generally, R is 1 ~ 2k, and C is 2.2 ~ 47uf.

b. The input impedance of CMOS is very high and is easily affected by induction, so the unused ends should be grounded or connected to a positive power supply when in use.

Since most of the energy exchange is also mainly concentrated on the power and ground pins of the device, and these pins are directly connected to the ground plane independently, the voltage fluctuation is actually mainly caused by the unreasonable distribution of current. However, the unreasonable distribution of current is mainly caused by a large number of vias and isolation.

The voltage fluctuation in this case will mainly transmit and affect the power supply and ground pins of the device. In order to reduce the instantaneous overshoot of the voltage on the power supply of the integrated circuit chip, decoupling capacitors should be added to the integrated circuit chip. This can effectively remove the influence of the burrs on the power supply and reduce the radiation of the power supply loop on the printed circuit board.

When the decoupling capacitor is connected directly to the power leg of the integrated circuit instead of to the power layer, the effect of smoothing the burr is the best. This is why some device sockets have decoupling capacitors, while some devices require the distance between the decoupling capacitor and the device to be small enough.

The general principles of decoupling capacitor configuration are as follows:

● Connect a 10~100uF electrolytic capacitor across the power input terminal. If the location of the printed circuit board allows, the anti-interference effect will be better if an electrolytic capacitor of 100uF or above is used.

● Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board is too small to fit, you can configure a 1-10uF tantalum electrolytic capacitor for every 4-10 chips. This device has a very small high-frequency impedance, less than 1Ω in the range of 500kHz-20MHz, and a very small leakage current (less than 0.5uA).

● For devices with weak noise resistance, large current changes when turned off, and storage devices such as ROM and RAM, a decoupling capacitor should be directly connected between the chip's power line (Vcc) and ground line (GND).

● The leads of the decoupling capacitor cannot be too long, especially the high-frequency bypass capacitor cannot have leads.

● When there are contactors, relays, buttons and other components on the printed circuit board, they will generate large spark discharges when they are operated, and an RC circuit is required to absorb the discharge current. Generally, R is 1 ~ 2K and C is 2.2 ~ 47UF.

● The input impedance of CMOS is very high and is susceptible to induction, so the unused ends should be grounded or connected to a positive power supply when in use.

● When designing, you should determine the use of three types of decoupling capacitors: high frequency, low frequency and medium frequency. The medium frequency and low frequency decoupling capacitors can be determined according to the power consumption of the device and PCB, and can be selected as 47-1000uF and 470-3300uF respectively; the high frequency capacitor is calculated as: C=P/V*V*F.

● Each integrated circuit has a decoupling capacitor. A small high-frequency bypass capacitor should be added next to each electrolytic capacitor.

● Use large-capacity tantalum capacitors or polycool capacitors instead of electrolytic capacitors as circuit charging and discharging energy storage capacitors. When using tubular capacitors, the outer shell must be grounded.

1.14.2. Configuration capacitor experience value

Good high-frequency decoupling capacitors can remove high-frequency components up to 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing a printed circuit board, a decoupling capacitor should be added between the power supply and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, providing and absorbing the charging and discharging energy of the integrated circuit when it opens and closes; on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitor in the digital circuit is a 0.1uf decoupling capacitor with a distributed inductance of 5nH. Its parallel resonance frequency is about 7MHz, which means that it has a good decoupling effect on noise below 10MHz and has almost no effect on noise above 40MHz.

1uf, 10uf capacitors, parallel resonance frequency above 20MHz, the effect of removing high frequency noise is better. It is often beneficial to put a 1uf or 10uf high frequency capacitor where the power enters the printed circuit board. Even battery-powered systems need this capacitor. For every 10 or so integrated circuits, add a charging and discharging capacitor, or called a storage capacitor. The capacitance size can be 10uf. It is best not to use electrolytic capacitors. Electrolytic capacitors are two layers of thin film rolled up. This kind of

The rolled-up structure behaves as an inductor at high frequencies, so it is best to use a bile capacitor or polycarbonate capacitor. The selection of the decoupling capacitor value is not strict, and can be calculated as C=1/f; that is, 0.1uf is used for 10MHz. Since no matter what power distribution scheme is used, the entire system will generate enough noise to cause problems, additional filtering measures are necessary. This task is completed by the bypass capacitor. Generally speaking, a 1uf-10uf capacitor will be placed at the power input end of the system, and a 0.01uf-0.1uf capacitor should be placed between the power pin and the ground pin of each device on the board. The bypass capacitor is a filter. The large capacitor (about 10uf) placed at the power input end is used to filter the low frequencies generated by the board (such as 60hz line frequency). The noise generated by the working devices on the board will produce harmonics from 100mhz to higher frequencies. Bypass capacitors should be placed between each chip. These capacitors are relatively small, about 0.1u. Capacitors are one of the most basic components in the circuit. Using capacitors to filter high-frequency interference on the circuit and decouple the power supply is familiar to all circuit designers. However, as the electromagnetic interference problem becomes more and more prominent, especially the increasing interference frequency, the prediction cannot be achieved due to the lack of understanding of the basic characteristics of capacitors.

This article introduces some easily overlooked parameters that affect capacitor filtering performance and matters that need to be paid attention to when using capacitors to suppress electromagnetic interference.

The role of capacitor leads

When using capacitors to suppress electromagnetic disturbances, the most easily overlooked problem is the effect of capacitor leads on the filtering effect. The capacitive reactance of a capacitor is inversely proportional to the frequency. It is precisely by utilizing this characteristic that capacitors are connected in parallel between the signal line and the ground line to bypass high-frequency noise. However, in actual projects, many people find that this method does not achieve the expected effect of filtering out noise, and they are helpless in the face of stubborn electromagnetic noise. One reason for this is that the effect of capacitor leads on the bypass effect is ignored.

The circuit model of an actual capacitor is shown in Figure 1, which is a series network consisting of equivalent inductance (ESL), capacitance, and equivalent resistance (ESR).

The impedance of an ideal capacitor decreases as the frequency increases, while the impedance of an actual capacitor is the impedance characteristic of the network shown in Figure 1. When the frequency is low, it presents a capacitor characteristic, that is, the impedance decreases as the frequency increases, and resonance occurs at a certain point, at which the impedance of the capacitor is equal to the equivalent series resistance ESR. Above the resonance point, due to the effect of ESL, the impedance of the capacitor increases as the frequency increases, which is the impedance characteristic of the capacitor showing an inductor. Above the resonance point, due to the increase in the impedance of the capacitor, the bypass effect on high-frequency noise is weakened or even disappears.

The resonant frequency of a capacitor is determined by ESL and C. The larger the capacitance or inductance, the lower the resonant frequency, that is, the worse the high-frequency filtering effect of the capacitor. In addition to being related to the type of capacitor, the lead length of the capacitor is also a very important parameter. The longer the lead, the greater the inductance and the lower the resonant frequency of the capacitor. Therefore, in actual engineering, the lead of the capacitor should be as short as possible. The correct and incorrect installation methods of the capacitor are shown in Figure 2.

0 ohm resistor effect

1. It has no function in the circuit, but is only used on the PCB for debugging convenience or compatible design.

2. It can be used as a jumper. If a certain line is not used, just don't attach the resistor (it will not affect the appearance)

3. When the matching circuit parameters are uncertain, use 0 ohms instead. During actual debugging, determine the parameters and then replace them with components with specific values.

4. When you want to measure the current consumption of a certain part of the circuit, you can remove the 0ohm resistor and connect the ammeter, which makes it convenient to measure the current consumption.

5. When wiring, if it is really difficult to route, you can also add a 0 ohm resistor

6. Under high-frequency signals, it acts as an inductor or capacitor. (It is related to the characteristics of the external circuit.) The inductor is mainly used to solve EMC problems. For example, between ground and ground, between power supply and IC Pin.

7. Single-point grounding (protective grounding, working grounding, and DC grounding are separated from each other on the equipment, each becoming an independent system.)

8. Fuse function

Single point grounding for analog and digital grounds#

*Single point grounding for analog and digital grounds*

As long as it is ground, it will eventually be connected together and then into the earth. If it is not connected together, it is a "floating ground", there is a voltage difference, it is easy to accumulate charge and cause static electricity. The ground is a reference to the 0 potential, all voltages are obtained with reference to the ground, and the ground standard must be consistent, so various grounds should be short-circuited together. People believe that the earth can absorb all charges and always maintain stability, and is the final ground reference point. Although some boards are not connected to the ground, the power plant is connected to the ground, and the power on the board will eventually return to the power plant and enter the ground. If the analog ground and the digital ground are directly connected over a large area, it will cause mutual interference. It is not appropriate not to short-circuit, and the reasons are as follows. There are four ways to solve this problem:

1. Connect with magnetic beads;

2. Connect with capacitor;

3. Connect with inductance;

4. Connect with 0 ohm resistor.

The equivalent circuit of the magnetic bead is equivalent to a band-stop limiter, which only has a significant suppression effect on the noise at a certain frequency. When using it, it is necessary to estimate the noise frequency in advance in order to select the appropriate model. For situations where the frequency is uncertain or unpredictable, magnetic beads are not suitable.

The capacitor blocks the direct current and the alternating current, causing the floating ground.

The inductor is large, has many stray parameters, and is unstable.

0 ohm resistor is equivalent to a very narrow current path, which can effectively limit the loop current and suppress the noise. The resistor has an attenuation effect in all frequency bands (0 ohm resistor also has impedance), which is stronger than the magnetic bead.

*Used for current loop when jumpered*

When the ground plane is split, the shortest return path of the signal is broken. At this time, the signal loop has to take a detour, forming a large loop area. The influence of the electric and magnetic fields becomes stronger, which is easy to interfere/be interfered. Connecting a 0 ohm resistor across the split area can provide a shorter return path and reduce interference.

*Configuration Circuit*

Generally, there should be no jumpers or DIP switches on the product. Sometimes users will mess with the settings, which may cause misunderstandings. In order to reduce maintenance costs, 0 ohm resistors should be used instead of jumpers and soldered on the board.

The vacant jumper is equivalent to an antenna at high frequencies, and the chip resistor has a good effect.

*Other Uses*

Cross-wire during wiring, temporary replacement of other SMD components for debugging/testing, as a temperature compensation device

Use decoupling capacitors well. Good high-frequency decoupling capacitors can remove high-frequency components up to 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing a printed circuit board, a decoupling capacitor should be added between the power supply and ground of each integrated circuit. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, providing and absorbing the charging and discharging energy of the integrated circuit when it opens and closes; on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitor in the digital circuit is a 0.1uf decoupling capacitor with a distributed inductance of 5nH. Its parallel resonance frequency is about 7MHz, which means that it has a good decoupling effect on noise below 10MHz and has almost no effect on noise above 40MHz.

1uf, 10uf capacitors, parallel resonance frequency above 20MHz, are better at removing high-frequency noise. It is often beneficial to have a 1uf or 10uf high-frequency removal capacitor where the power enters the printed circuit board, even battery-powered systems need this capacitor. For every 10 or so integrated circuits, add a charging and discharging capacitor, or storage capacitor, and the capacitance can be 10uf. It is best not to use electrolytic capacitors, which are two layers of thin film rolled up. This rolled-up structure behaves as an inductor at high frequencies. It is best to use a bile capacitor or polycarbonate capacitor.

The selection of decoupling capacitor value is not strict, and can be calculated as C=1/f; that is, 0.1uf is used for 10MHz, and for a system composed of a microcontroller, the value can be between 0.1~0.01uf.

3. Some experience in reducing noise and electromagnetic interference.

(1) If a low-speed chip can be used, don’t use a high-speed chip. High-speed chips are used in critical places.

(2) A resistor can be connected in series to reduce the rate at which the control circuit rises and falls.

(3) Try to provide some form of damping for relays etc.

(4) Use the lowest frequency clock that meets the system requirements.

(5) The clock generator should be as close as possible to the device that uses the clock. The shell of the quartz crystal oscillator should be grounded. (6) Use the ground wire to circle the clock area and keep the clock line as short as possible.

(7) The I/O drive circuit should be as close to the edge of the printed circuit board as possible, so that it can leave the printed circuit board as soon as possible. The signals entering the printed circuit board should be filtered, and the signals coming from the high noise area should also be filtered. At the same time, the terminal resistor should be used in series to reduce signal reflection.