压制EMI噪声的技术

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1、Techniques To Suppress EMI Noise By Frany yang2004.2Rev 1.01 The Contents 1.Noise Sources in Digital Equipment 2.Suppressing EMI Noise Emission 3.How to Select and Use EMI Suppression Filters 4.Conclusions2 1.1 Digital signal and harmonic Components 1.2 Radiated Noise from Digital Circuit Boards 1.3

2、 Radiated Noise from Cable 1.4 Causes of Common Mode Noise3 Digital signals are rectangular waves made up of a large number of sine waves.Digital signals waveform contains fundamental waves whose frequency is the signals cycling frequency and harmonic waves whose frequencies are integer-multiples of

3、 the fundamental waves frequency.4 When the duty ratio is 50%,only the odd order harmonics are included;otherwise,some even order harmonics are also included.5 1.2.1 Noise generated by IC 1.2.2 Radiated noise from patterns 1.2.3 Effect of EMI suppression Filter 6Radiation of noise requires both a no

4、ise source where noise is generated and antenna from which noise can be received.In this experiment,the circuit board contained noise source ICs but no antenna.7 A pattern that will act as the antenna forradiated noise is connected to the noise source IC2.8 an EMI suppression filter to suppress nois

5、e that is radiated from a signal pattern acting as an antenna.9 cable is more efficient in radiating noise.Suppression method10IC4 is added as the interface circuit,a 1 m long signal cable and a grounding cable are connected to its output.11Suppression method:1213 When a signals return current flows

6、 to ground,the ground patterns inductance causes a potential difference developed between the grounds of IC1 and IC2.It is possible that the noise caused by this potential difference flows through the grounds and the interface circuit to the cable.The common mode noise generated in this manner can b

7、e effectively suppressed by reducing the ground impedance,suppressing the signal current,or inserting an EMI suppression filter in the signal line.14 2.1 Approaches to Suppression Emission of EMI Noise 2.2 Principal Emission Suppressing Methods 2.3 EMI Suppression Filters 2.4 Improved Ground Patten

8、2.5 Changing Component and Pattern Layout 2.6 Shielding15In typical electronics circuits,noise is radiated from both circuit patterns and connecting cables.16 EMI noise emission suppression model17Guidelines to select noise suppressing point Level of direct emission Strong-Weak from noise source Num

9、ber of noise Many-Few transfer routes(cables)Number of noise Few-Many sources(ICs)Suppress noise sourceSuppress noise at transfer route1819 It is important to thinking about how to use EMI suppression filters from the design stage and not when the prototype board is completed.More effective noise su

10、ppression can be achieved by installing the EMI filters at the potential noise source and conductive noise paths during the initial design stage and then removing unnecessary filters.20 The basic rule of EMI suppression filter application is to locate it near the noise source or the noise receiving

11、unit so that the effectiveness will not be degraded by coupling of the filters input and output wiring.When applying EMI suppression filters on a cable,the circuit requiring the noise suppression should be considered the noise source or the noise receiving unit and the filter is located at the root

12、of the cable.21 2.3.1 Use Inductor Type EMI Suppression Filter 2.3.2 Use Capacitor Type EMI Suppression Filter22 Example:Ferrite bead inductor Inductor type EMI suppression filters are inserted in series with the signal path.The inductor type suppression filter generates impedance that is primarily

13、resistive at the high-frequency band where EMI noise becomes problem and uses this resistance to absorb noise.23 Examples:Three-terminal capacitors,EMI suppression filters for signal lines Capacitor type EMI suppression filter is inserted between the line through which noise is transferring and the

14、ground.The filters ground terminal is attached as follows:24(a)When used at noise source Filters ground terminal is connected to the noise source sides ground,this enables bypassing of normal mode noise to the ground.(b)When used on conductive noise path The common mode noise is the problem,capacito

15、r type EMI suppression filters ground terminal is connected to a ground that is very stable.25used at the noise source26used at the noise source27used on conductive noise path28used on conductive noise path29Ground impedance is reduced by making the ground patternbetween the signal ICs input and out

16、put wide and short.This minimizes the potential difference relative to the ground30 Common mode noise can be minimized by using a wider and shorter ground pattern between the signal input and output ICs to reduce the ground impedance31Improve ground pattern with ground plane32 Component and pattern

17、layout should be done by giving priority to shortening the length of high-speed signal patterns.Common mode noise that is generated by or emitted from signal patterns is stronger when the signal frequency is higher and the signal path length is longer.33 Cable should be located away from high-speed

18、signal circuit to minimize noise coupling.34 Examples of how signal frequency influences noise levels.The faster the signal frequency,the higher the noise levels extend to higher frequency ranges.Influence of signal frequency 8MHz 16MHz 25MHz35The path length caninfluence radiated noise.Frequency ba

19、nd of noisethat is radiated changes with the pattern length L.Normally,minimizing the pattern length results in lowering of overall noise radiation level.36 L=20cmL=10cmL=5cmInfluence of signal path length37 Principle of shieldingThe principle of shielding effect is produced by reflection and absorp

20、tion of unwanted noise.Reflection is the dominant factor for high-frequency band as is present in electromagnetic field radiation test.The basic shielding materials for this frequency band is aluminum or comparable low impedance material.38 Making the shield cases openings smaller in size and in int

21、ervals Heat ventilating openings that consist of interspersed smaller holes will provide better shielding effect.39)2log(20lSshielding effectiveness403.1 EMI Suppression Filter performance and Signal Waveform3.2 Circuit Impedance and EMI Suppression Filters Performance3.3 Selecting Capacitor Type or

22、 Inductor Type EMI Suppression Filter3.4 Examples of EMI Suppression Filter Use at Noise Source 3.5 Examples of EMI Suppression Filter Use on Conductive Noise Path 41 In addition to suppression noise,EMI filters may cause distortion of signal waveform.When the capacitance value of three-terminal cap

23、acitor is increased,impedance of inductor is raised,or cutoff frequency of EMI suppression filter for signal lines is lowered,the noise suppression performance is increase but the distortion of signal waveform also increase.4243 Insertion loss of EMI filter varies depending on thecircuit impedance.4

24、4 1.At Noise Source(a)Capacitor type EMI suppression filter as primary device Line with high circuit input or output impedance Line with high noise level(Ex.Clock line;control bus line)(b)Inductor type EMI suppression filter as primary device Line with low circuit input or output impedance(Ex.Power

25、supply line to which bus controller is connected)Line with relatively low noise level(Because filter grounding is unnecessary,and installation is simple)Line requiring current control(Ex.Multiple lines that switch simultaneously and in which large current flow to the ground:Address/data bus;control

26、bus)45 2.Noise on conductive path Generally,both capacitor type and inductor type filters are used in combination for suppression noise in a conductive noise path.When noise level cannot be reduce adequately by using filters in conductive path alone,filters must be used at the noise source as well.4

27、6 Bus lines contain many lines that switch on and off simultaneously.Especially on data and address bus lines,an instantaneous large current flows into the GND and power supply lines.Therefore,it is necessary to suppress the current flow on the signal lines.47 Current is suppressed mainly by install

28、ing the inductor type EMI filter in the oscillator circuit output,when necessary,capacitor type EMI filter is used in combination.48 For EMI noise suppression where signal cables are connected,chip solid EMI filter,a three-terminal capacitor product,and chip Ferrite Bead Inductor are used in combina

29、tion.49 Both normal mode and common mode noises exists in power supply line.(in a circuit with relatively stable ground,normal mode noise is the primary noise,in a circuit with unstable ground,common mode noise is also present.)50 Describe various EMI noise source types from simple experiments using

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