First, EMC engineers must have eight skills

EMC engineers need those skills? From the enterprise product design, rectification certification process, EMC engineers must have the following eight skills:

1, EMC's basic test items and test process mastery;

2, the product corresponds to the standard EMC grasp;

3, product positioning of the rectification of EMC grasp;

4, a variety of product certification process master;

5, product hardware and software knowledge of the circuit (master, interface) to understand;

6, EMC design rectification components (capacitors, beads, filters, inductors, transient suppression devices, etc.) to master;

7, mastery of product structure design skills mastery;

8, EMC design how to intervene in product development process of various stages of mastery.

Second, EMC common components introduced

Common mode inductance

Since EMC is mostly solving common mode interference problems, common-mode inductors are also one of the most commonly used powerful components! Here to give you a brief introduction to the principle of common mode inductance and the use of.

Common-mode inductance is a common-mode interference suppression device with ferrite core. It consists of two symmetrically wound coils of the same size with the same number of turns wound on the same ferrite toroidal core to form a four-terminal The device, which has an inhibitory effect on the common mode signal exhibiting a large inductance, has little effect on the differential mode signal showing a small leakage inductance. The principle is that when the common mode current flows, the magnetic fluxes in the magnetic ring are superposed with each other, so that there is a considerable amount of inductance which can restrain the common mode current. When the two coils flow through the differential mode current, the magnetic Through offset each other, almost no inductance, so the differential mode current can be passed without attenuation. Therefore, the common-mode inductance in the balanced line can effectively suppress the common-mode interference signal, and the normal transmission line differential-mode signal has no effect.

Common mode inductors in the production should meet the following requirements:

1) Wound around the core of the coil wire to be insulated from each other to ensure that the instantaneous over-voltage under the interrupting of the coil does not occur breakdown short circuit.

2) Do not saturate the core when the coil flows through an instantaneous high current.

3) The core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage.

4) The coil should be wound as much as possible, so as to reduce the stray capacitance of the coil and enhance the ability of the coil to instantaneous over-voltage.

Under normal circumstances, at the same time pay attention to select the desired filter frequency band, the larger the better the common-mode impedance, so we need to look at the device information in the choice of common-mode inductor, mainly based on impedance frequency curve selection. In addition, pay attention to consider the choice of differential mode impedance of the signal, the main concern of differential mode impedance, with particular attention to high-speed port.

Magnetic beads

In the digital circuit EMC product design process, we often use the beads, then the principle of the bead filter and how to use it?

Ferrite material is ferro-magnesium alloy or iron-nickel alloy, this material has a high permeability, he can be inductance between the coil windings in the case of high-frequency high-impedance capacitor to produce the smallest. Ferrite materials are commonly used at high frequencies because their main pass inductance behavior at low frequencies minimizes on-line losses. At high frequencies, they predominate as a ratio of reactance and change with frequency. In practice, ferrite materials are used as RF attenuators for RF circuits. In fact, the ferrite is better equivalent to the resistance and inductance in parallel, low frequency inductance is shorted by inductance, inductance inductance becomes very high at high frequency, so that all the current through the resistor. Ferrite is a dissipating device where high-frequency energy is converted into heat energy, which is determined by his resistance characteristics.

Ferrite beads have better high frequency filtering characteristics than ordinary inductors. Ferrite is resistive at high frequencies, which is equivalent to a low quality factor inductor, so it maintains a high impedance over a wide frequency range, improving high frequency filtering performance. In the low frequency range, the impedance is composed of the inductance of the inductor, R is small at low frequencies, and the magnetic permeability of the core is high. Therefore, the inductance is large, L plays a major role, and the electromagnetic interference is reflected and suppressed. The core is less lossy, and the entire device is a low-loss, high-Q inductor. This inductor tends to resonate, so there are times when the use of ferrite beads can sometimes lead to increased interference at lower frequencies. In the high frequency band, the impedance consists of a resistive component. As the frequency increases, the magnetic permeability of the core decreases, causing the inductance of the inductor to decrease and the inductance component to decrease. However, when the loss of the magnetic core is increased and the resistance component is increased, the total impedance is increased. When the high-frequency signal passes through the ferrite, the electromagnetic interference is absorbed and converted into heat and dissipated.

Ferrite suppression components are widely used in printed circuit boards, power lines and data lines. Such as the printed circuit board power input with ferrite suppression element, you can filter out high-frequency interference. Ferrite beads or magnetic beads are specifically designed to suppress high frequency interference and spikes on signal lines and power lines. They also have the ability to absorb pulsations of electrostatic discharge.

The use of chip beads or chip inductors mainly lies in the practical application. Chip resonators are needed in the resonant circuit. The need to eliminate unwanted EMI noise, the use of chip beads is the best choice. Chip Beads and Chip Inductors Applications: Chip Inductors: RF and Wireless Communications, Information Technology Equipment, Radar Detectors, Automotive Electronics, Cellular Phones, Pagers, Audio Devices, PDAs (Personal Digital Assistants) Wireless remote control system and low-voltage power supply module. Chip Beads: Clock Generation Circuits, Filters Between Analog Circuits and Digital Circuits, I / O In / Out Internal Connectors (Serial, Parallel, Keyboard, Mouse, Long Distance, Local Area Network) Noise immunity in computers, printers, video tape recorders (VCRS), television systems and mobile phones, between high-frequency conducted interference and susceptible logic devices, power circuits to eliminate high frequency conducted interference.

The unit of the bead is ohm, because the unit of the bead is nominally based on the impedance it produces at a certain frequency, and the unit of impedance is also ohm. Beads DATASHEET generally provide the frequency and impedance characteristics of the graph, generally 100MHz as the standard, such as the magnetic beads at 100MHz frequency equivalent to 1000 ohms. The frequency band that we want to filter needs to select the bigger the impedance of the magnetic bead is better, usually choose the impedance of 600 ohms above.

In addition, you need to pay attention to the magnetic beads flow through the beads, the general need to derate 80% of the treatment, when used in the power circuit to consider the impact of DC impedance on the pressure drop.

Filter capacitor

Although the resonance of a capacitor is undesirable from the standpoint of filtering out high frequency noise, the resonance of the capacitor is not always detrimental. When the frequency of the noise to be filtered out is determined, the capacitance of the capacitor can be adjusted so that the resonance point falls just on the frequency of the disturbance.

In practical engineering, the frequency of electromagnetic noise to be filtered often reaches hundreds of MHz or even more than 1 GHz. For such high-frequency electromagnetic noise must use through-core capacitance can be effectively filtered. The reason why ordinary capacitors can not effectively filter out high-frequency noise is due to two reasons, one is that capacitor leads inductances cause capacitance resonance, high impedance signals appear on the high-frequency signals, which impairs the bypass effect on high-frequency signals; Another reason is the parasitic capacitance between the wires coupling the high-frequency signals, reducing the filtering effect.

The reason why the through-core capacitor can effectively filter out high-frequency noise is that the through-core capacitor not only has no lead inductance, but also has the problem that the resonant frequency of the capacitor is too low, and the through-core capacitor can be directly mounted on the metal panel, Frequency isolation effect. However, when using through-core capacitors, the problem to be aware of is the installation problem. The weakest point is to wear the greatest capacitance of the heart is afraid of high temperature and temperature shocks, which will pass through the capacitor to the metal panel caused great difficulties. Many capacitors break down during soldering. Especially when a large number of feedthrough capacitors need to be mounted on the panel, as long as there is a damage, it is very difficult to repair, because the damage will be caused when other capacitors are damaged when the damaged capacitor is removed.

With the increase of the complexity of electronic equipment, the hybrid equipment of internal and external power is mixedly installed, and digital logic circuits are more and more mixedly installed. The mutual harassment between circuit modules becomes a serious problem. One of the ways to solve the mutual harassment of such circuit modules is to use metal cages to isolate circuits of different nature. However, all the conductors passing through the cofferdam should pass through the through-core capacitor, otherwise the isolation will be invalidated. When there is a large number of interconnections between different circuit modules, it is very difficult to install a large number of through-core capacitors in the bulkhead. In order to solve this problem, many foreign companies have developed "filter array board", which is a special process prior to the through-core capacitance welding in a metal plate consisting of devices, the use of filter array board can easily solve a large number of wires through the metal panel problem. However, the price of this filter array board is often higher, the price of each pin is about 30 yuan.

Third, EMI / EMC design classic 85 asked

1, why do electromagnetic compatibility design of the product?

A: Meet the product function requirements, reduce commissioning time, the product meets the requirements of the electromagnetic compatibility standard, so that the product will not produce electromagnetic interference to other devices in the system.

2, the product of electromagnetic compatibility design can be carried out from several aspects?

A: Circuit Design (including device selection), software design, circuit board design, shielding structure, signal / power line filtering, circuit grounding design.

3, in the field of electromagnetic compatibility, why always with decibel (dB) unit description?

A: Because of the wide range of amplitudes and frequencies to be described, it is easier to represent logarithmic coordinates on the graph, and dB is the unit of logarithm.

4, EMC, I know little, but now the circuit design faster and faster data transfer, I PCB system in the manufacturing process, but also encountered some PCB EMC problems, but feel too submerged. I would like to study and learn well in this area, not with the crowd, we learn what I learn, is that I really think EMC in the future circuit design is more and more important, as I said earlier, Do not know their own deep, do not know how to start, would like to ask, to do more in the EMC area, what are the basic knowledge needed, what basic courses should be learned. How to learn is a relatively good way. I know that it is not easy for any one to learn well and have never thought of engaging him in a short period of time. I just hope to give some suggestions and try our best not to take any detours.

A: About EMC, you should first understand EMC standards such as EN55022 (GB9254), EN55024, and simple test principle. In addition, you need to understand the use of EMI components such as capacitors, beads, differential mode inductors, common mode inductors, At the PCB level need to understand the layout of the PCB, the laminated structure, the impact of high-speed wiring on the EMC and some rules. Another point is that EMC problems need to grasp some analysis and solution ideas. These are the basic knowledge that must be mastered as a hardware person in the future!

5, I am a novice just involved in PCB design, I would like to ask you to consult, in order to make a good PCB design I should grasp a lot of knowledge? In addition, PCB design encountered in the safety and general aspects of the general knowledge Where can I find? Hope your advice, be grateful!

A: The PCB design should be mastered:

1, Familiar with the relevant PCB design software, such as POWERPCB / CANDENCE;

2, to understand the specific structure of the familiar product design, familiar with the schematic circuit knowledge, including digital and analog knowledge;

3, master PCB processing, process, maintenance requirements can be processed;

4, master PCB board high-speed signal integrity, electromagnetic compatibility (emi and ems), SI, PI simulation design and other related knowledge;

5, if the relevant work involves radio frequency, but also need to grasp the radio frequency knowledge;

6, according to the PCB design to the knowledge of the main look GB4943 or UL60950, the general insulation spacing requirements can be obtained through the look-up table!