Collection | combined with my personal experience, I have summarized these seven EMC related knowledge

Hello everyone , I'm a snail . Review several projects this year EMC Over and over again, it still took a lot of time . Combine your own experience , Share relevant experience with you . Welcome to correct and add .

1. Time is money

A complete set of EMC It may cost millions to build the test environment , Especially in the radiation testing environment like darkroom , General enterprises do not have this condition . So they usually go to certification bodies or large enterprises EMC test .

Tests are charged by the hour , The charging standard is different in every place . Like our side, it is generally 1000 element / Hours , Regular customers' words , You can bargain .

Timing is calculated according to the beginning and end , Intermediate rectification time is also included . So when the test fails , We must come up with rectification measures in time . It is recommended to test at most 2-3 Hours , Otherwise it will take a long time , The train of thought is getting more and more dizzy , More and more disorderly .

If the problem is not solved , When you go back, you must record the experimental process immediately , What rectification measures have been taken , What are the next rectification plans . Because some details may really be forgotten after a long time .

2. Tools must Get ready complete

Certification bodies don't have everything , It's not just taking the prototype directly to test . Many things need to be prepared by yourself . It is recommended that you take these things ：

Tweezers , Adhesive tape , Copper foil , Resistance book , Capacitance book , Various X capacitance 、Y Capacitance and common mode coil , There are also relevant test tooling .

I can think of it anyway , Try to bring it , Otherwise, you may pay a high price for forgetting to take a gadget .

3. The connecting line should be as short as possible , And route from the edge of the board .

Generally speaking , Low frequency interference , It spreads through conduction , High frequency interference , Through space wireless transmission .

The flat cable is equivalent to an antenna , The radiated electromagnetic energy of other circuits in the chassis is induced to the flat cable , Generate voltage and current , It causes radiation in the wiring .

The higher the frequency of the interference signal , The longer the connecting cable , The higher the efficiency of transmitting interference , The greater the external interference . When the length of the connecting line is the wavelength corresponding to the interference signal 1/4 perhaps 1/4 An odd number of times , The interference emission efficiency is the highest .（λ=v/f）

image wifi The antenna ,4G The antenna is usually on the edge of the board , Because the antenna will be like an external signal , That's why . therefore , The connecting cable is usually on the edge of the board , And as short as possible , Try to fix it with a tie .

If the wiring is disturbed , Select shielded wire , The effect may be better . Or put a magnetic ring on the line , But this disadvantage is inconvenient to produce .

4. Interference caused by clock signal and its solution

The clock signal is a periodic signal , Generally, the frequency is relatively high , And it's a square wave , The rising edge time is steep , The energy is relatively strong , Will produce a lot of radiation . Usually, the radiation at the frequency doubling point exceeds the standard, which will be more serious .

In this year's project , I once encountered a clock frequency exceeding the standard , But the solution cycle is relatively long .

① It is suspected that there is a problem of laying copper on the ground over the wiring , Change the board once ;

② Suspected of buying fake chips , It's a quality problem , Changed chip manufacturers twice ;

③ Try shielding with copper foil , The effect is not ideal .

Last , We try it on the clock signal Magnetic beads + capacitance or resistance + Capacitance matching The way . The purpose is to change the signal quality , Make the waveform slow , After matching, the signal will be greatly improved , Radiation will also drop a lot . But there's a problem , To what extent ？ First of all, we must ensure the normal operation of the equipment , Then it needs to be tested and verified many times .

A friend recommended the spread frequency crystal oscillator before , Its principle is to modulate the peak clock , Change it from a narrow-band clock to a spectrum with sidebands , The peak energy is dispersed to multiple frequency bands in the spread-spectrum region , So as to reduce peak energy , Inhibition EMI The effect of .

Sounds good . Due to the time node of the project and the price of crystal , This scheme was not used at that time , If you have any useful friends, please let me know .

  

5. Generation and solution of differential mode interference

Differential mode interference exists in L-N Between lines , The current flows from L Get into , Flow through the positive pole of rectifier diode , And then flow through the load , Back to N. On this path , High power devices with high-speed switches , Diode with very short reverse recovery time , The high frequency interference produced by these devices , Will flow through the whole circuit , Thus, it is detected by the receiver , Lead to excessive conduction .

The rectification of differential mode interference has the following methods ：

1. On the hot line L And zero line N Set a magnetic ring between

2. increase X Capacitance value

3. Increase the common mode inductance , Use its leakage inductance , Suppress differential mode noise

6. Generation of common mode interference and terms of settlement

Common mode interference exists in the parasitic capacitance between the power input line and the earth , The interference noise flows to the earth through the capacitor , stay LISN- power cord - Parasitic capacitance - Common mode interference current is formed between the ground , Thus, it is detected by the receiver , Lead to excessive conduction .

There are the following methods to rectify common mode interference ：

1. Put a magnetic ring on the ground wire .

2. Increase the inductance of common mode inductance .

3. The main board shall be grounded as far as possible , Reduce the impedance to ground , So as to reduce the parasitic capacitance between the cable and the earth .

7.BUCK Conducted interference caused by power supply Of terms of settlement

In the circuit , We often use BUCK chip , Will input +12V Turn into +5V or +3.3V Power other chips .BUCK The chip is a switching power supply , Its switching frequency will affect the conducted radiation, resulting in failure , This is a very common question .

In addition to strictly referring to the chip manual for layout , Painting PCB Pay more attention to copper coating and ground covering , Pay special attention to keep the ground circuit in good condition . Besides , Here are three solutions ：

First of all , Input FireWire L String one 10R Left and right power resistors , In addition to the good improvement of conduction , It can also suppress surge current .

second , Refer to my previous article 《 In order to prevent BUCK Influence of power supply on conduction test , I added π Type filtering , But the inductance starts screaming again ...（ Click the title to enter the view ）

Third , Refer to my previous article 《 dried food | Share a EMC Actual cases and rectification process 》（ Click the title to enter the view ）

Last , Usually know more EMC Related knowledge , Familiar with relevant standards , In designing circuits , When drawing a board, you should EMC Take into account .EMC The earlier the question is considered 、 The sooner you solve , The smaller the cost 、 The better the result. .

Recommend a link to find standard documents ：http://www.upbz.net/index.html

That's my summary , I hope it helps you .

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