EMC rectification ideas

First look at the definition . The definition is mainly based on the nature of the problem , It is divided into 【 Power Supply 、 The clock CLK problem 、 Ground imbalance 】.

Let's look at the cause analysis ：

For three problems , Xiaobian has examples to analyze ：

Look at the power supply first ：

1. Investigation means

2. Problem analysis

The general power problem is DC-DC Circuit induced devices （dcdc chip 、 inductance 、 diode ） Type selection ：

The general power problem is DC-DC Caused by a circuit dcdc Power Supply PCB Some unreasonable design problems ：

3. root

Look at the clock again ：

Traditional solutions and traditional means in solving ideas ：

Hardware spread spectrum ：

The replacement scheme in the solution idea ：

Ground imbalance :

Last , Analysis methods ：

EMC Three rules

1） Rule one 、EMC The law of cost efficiency ratio ：EMC The earlier the question is considered 、 The sooner you solve , The smaller the cost 、 The better the result. .

In the new product development stage EMC Design , Than wait until the product EMC Only when the test is unqualified can the improvement be carried out , The cost can be greatly saved , Efficiency can be greatly improved ; conversely , Efficiency will be greatly reduced , The cost will increase greatly .

Experience tells us , At the same time of function design EMC Design , To template 、 When the prototype is completed, it passes EMC test , Is the most time-saving and economic . contrary , Product development stage does not consider EMC, It was found that EMC Only when it is unqualified can it be improved , It's not just technically difficult 、 And rework will inevitably lead to a great waste of cost and time , Even because of the structural design 、PCB Design flaws , Unable to implement improvement measures , The product cannot be put on the market .

2） Rule two 、 High frequency current loop area S The bigger it is , EMI The more radiation .

The current of high frequency signal flows through the minimum path of inductance . When the frequency is high , Generally, the reactance of wiring is greater than the resistance , The connection to the high frequency signal is the inductance , Series inductors cause radiation . Electromagnetic radiation is mostly EUT The high frequency current loop on the device under test produces , The worst case is the open circuit antenna . The corresponding solution is to reduce 、 Shorten the connection , Reduce the area of high frequency current circuit , Try to eliminate any antenna that is not required for normal operation , Such as discontinuous wiring or excessively long pins of components with antenna effect .

One of the most important tasks to reduce radiation disturbance or improve the anti-interference ability of RF radiation , Try to reduce the area of high frequency current loop S.

3） Rule three 、 Loop current frequency f The higher the , Caused by the EMI The more radiation , Electromagnetic radiation field strength with current frequency f The square of increases in proportion to .

Two of the most important ways to reduce radiation disturbance or improve the anti-interference ability of RF radiation , It is to try to reduce the frequency of high frequency current of disturbance source f, That is to reduce the frequency of the disturbance electromagnetic wave f.

The following content of this article , Is to use the above three laws , Advocate thinking ahead of time EMC problem , Introduce EMC Design and EMC Problem improvement .

improvement EMC problem , It's like treating a disease . If the product fails EMC test , We take the measurements , We can only know which frequency points “ excessive ” 了 , And where do these frequencies come from , It's often the least easy for engineers to find out 、 The most difficult problem to solve . product EMC problem , It's hard to say , It's easy to say .

improvement EMC problem , First , according to EMI The way and mechanism of production , That is to say EMC The elements of the problem , in the light of EUT（ The tested sample , The same below ） The circuit principle of , Make some judgments first , such as IT Class a equipment and AV Audio and video equipment EMC What is the cause of the problem or the source of internal harassment , Start with an inference , Combined with the test The project test chart looks at the essence through the phenomenon , Analyze the cause of out of tolerance -- Find out the source of harassment , Find out the way of harassment , In order to have a definite aim .

Analyze the cause of out of tolerance , High frequency oscilloscope or spectrum analyzer can be used Field probe verification analysis results , From frequency domain to time domain , From frequency domain to time domain , analysis 、 To seek to produce EMC The corresponding circuit and device of the problem .

EMC Three elements of the question

Switching power supply and digital equipment have rich high frequency harmonics due to pulse current and voltage , So there's a lot of radiation . Electromagnetic interference includes radiation type ( high frequency )EMI、 Conduction type ( Low frequency )EMI, That is, to produce EMC There are two main ways to solve the problem : One is the form of space electromagnetic interference ; The other is through conduction , let me put it another way , produce EMC The three elements of the question are ： Electromagnetic interference source 、 coupling The way 、 Sensitive equipment .

Radiation interference mainly pollutes the space electromagnetic environment in the form of electromagnetic wave through the shell and connecting wire ; Conducted interference is through the power line to disturb the public grid or through other terminals （ Such as ： RF terminals , Input terminals ） Affect connected devices .

Conduction 、 radiation 、 Source of harassment ------（ The way ）----- Near field coupling of sensitive receptors IT、AV Possible sources of harassment for the device

a） FM Receiver 、TV Receiver local oscillation , Fundamental wave and harmonic wave from the high frequency head 、 The local oscillation circuit is generated ;

b） Switching pulse and high harmonic of switching power supply , Synchronous signal square wave and high frequency harmonic , A line produced by a line scan imaging circuit 、 Field signal and high frequency harmonic ;

c） All kinds of clock signals and high frequency harmonics needed for digital circuit operation 、 And their combination , Various clocks such as CPU Chip working clock 、MPEG Decoder working clock 、 Video synchronization clock （27MHz,16.9344MHz ,40.5MHz） etc. ;

d） Digital signal square wave and high frequency harmonic , The higher harmonic produced by crystal oscillator , Nonlinear circuit phenomena （ Nonlinear distortion 、 Intermodulation 、 Saturation distortion 、 Cut off distortion ） Etc 、 Stray signals ;

e） Nonsinusoidal wave form , Wave burr 、 Overshoot 、 Ring the bell , Parasitic frequency points in circuit design .

f ） For sensitive receptors, external disturbances received through coupling include surges 、 Fast pulse train 、 Static electricity 、 Voltage drop 、 Voltage variation and various electromagnetic fields .