The importance of EMC

 

Be in early last centuries at the beginning of 80 time, american new jersey a hospital is obstetric the infantile mortality of ward area is quite tall. Late night, the caution light on the monitor of surveillant baby always is ground of for no reason at all go out. To this, nurses are very rusty, then they shut monitor, back and forth one by one is perambulatory.

 

After passing preliminary investigation one time, the professor found out the truth of this thing, the catapult of one TV station receives the permission of American communication committee near before, in power can output his to rise quite high after midnight about, but must be in in the morning before 6 o’clock, or the time that other appoints, regain original level. The join cable between the monitor of nurse station and every baby disturbs frequency place to produce resonance in these, inductive voltage and make monitor caution light goes out. The hospital is in before discovering this problem, already had about the same 6 children die.

 

Have a case again: A such client complains reaction, should be inside the computer room open when power source of a switch, the local area network of the 100M speed of this company appears rate falls and inactive phenomenon, and the network of 10M speed did not suffer an effect however. Put out power source, the network returns to normal.

 

Classics experiment discovers after, the lotus root of signal of high frequency interference of this switch power source combines network line, make the network appears breakdown.

 

The history that EMC develops

 

Develop at full speed and EMC is born what accompanying latter-day electron industry actually. To last century end, increase quickly as electron, electric equipment. EMC has expanded numerous domain, can none exaggerated says: Where to have electronic product, where to have EMC problem. Western country is more and more slashing also to this requirement, EMC already became product of developing country electron to enter one of commerce camp of western market.

 

Will tell to the enterprise, different EMC devises an idea, can bring about the waste on different cost and time.

 

 

The content of EMC

 

● basic idea:

 

) of compatibility of ★ EMC(electromagnetism: Electromagnetic Compatibility

★ EMI(electromagnetism is disturbed) : ELectromagnetic Interference

★ EMS(electromagnetism fights faze sex) : Electromagnetic 　 Susceptibility

★ ESD (electrostatic) : Electrostatic Discharges

★ RS (radiation interference rejection) : Radiated Susceptibility

★ EFT(report is fast twinkling changes pulse group) : Electronic Fast Transients

Surge of ★ SURGE(be struck by lightning)

★ CS(conducts interference rejection) : Conducted Susceptibility

 

 

● EMC =EMI +EMS

 

★ EMI = Conduction (Harmonic) + Radiation

★ EMI3 element: Systematic level is below, ask everybody to consider PCB level.

 

 

EMI of switch power source is discussed

 

The source that ● EMI generates:

 

★ the first, because of,the biggest drawback of switch power source is switch movement (TURN-ON or TURN OFF) generation noise voltage is its noise source. Because the weaveform of switch movement is square wave, and square wave is contained a lot of tall second harmonic. (Dv/dt)

★ the 2nd, as a result of switch report crystal nonlinear reach 2 extremely restore character of body reversely, electric current is made fast nonlinear change causes noise. (Di/dt)

 

 

The transmission way of ● EMI and way:

 

Signal of ★ EMI interference presses its character to be able to be divided for in all modular signal (COMMON MODE) with wrong standard signal (DIFFERENTIAL MODE) .

★ in all modular signal: The on the lead of two loop electric current way that violates signal electric current is opposite the earth is identical signal, call in all modular signal, see Zun Tu;

★ needs modular signal: The on the lead of two loop electric current way that violates signal electric current is opposite the earth is opposite signal, call wrong standard signal, see next right pictures.

 

 

● is commonly used of structure of the low wave that connect filter differentiate

 

 

● power source inputs the design of filter:

 

★ in all modular poor model is designed apart (with π model for exemple)

 

 

★ filter in all modular part design

 

 

★ filter differs modular part design

 

 

The installation of ● filter:

 

 

 

● in all of modular inductance circle make

 

 

In all the magnetic field cancel that the load current in modular choke produces, magnetism core won’t be accordingly saturated.

 

● magnetism bead impedance

 

 

Attention: In all modular inductance is mixed magnetism bead needs to measure temperature rise! !

 

Develop attacks EMI analyses citing

 

 

Flyback framework EMI is analysed

 

The high frequency and equivalent model of ● Flyback framework

 

 

● Noise source:

 

Big Di/dt and the place that Dv/dt produces, to Flyback framework, what can produce these change is main have:

 

★ transformer TX1;

★ MOSFET Q1;

★ outputs diode D1;

The RC oscillation of ★ chip;

★ drive line;

 

 

The weaveform of the Vds on Q1

 

The Noise of the generation when MOSFET movement: Institute of as above plan is shown, basically come from 3 respects:

 

① Mosfet is enlightened, when closing, have very wide spectrum content, the harmonic itself of switch frequency is stronger interference source.

The oscillation when ② closes the interference with 1 stronger generation.

The oscillation when ③ closes the interference with 2 stronger generation.

 

Switch is in charge of Q1 to close, deputy edge diode D1 guides when connecting (belt carry) , the excitation inductance of former edge by clamp down on, the energy of Lep of feeling of former edge leakage undertakes discharging through the parasitism capacitance Cds of Q1, advocate discharge loop is Lep—Cds—Rs—C1—Lep, the frequency that produces oscillation vibration right now is:

 

 

The alternate of oscillation voltage Vlep on Lep is added on 2Vc1, cause Vds=2Vc1+Vlep. Of oscillation lose by force, the stability of the compression value of the pipe that the decision we choose, circuit.

 

Measure Lep=6.1uH, q1 is 2611 check norms book to be able to get Coss=190pF (Coss is equal to Cds approximately) , and this charges board for two pipe paralell connection, so Cds=380pF. By on type can get F =3.3 MHz, with the oscillation frequency be identical in next graphs.

 

 

Can see from inside the graph this oscillation is the oscillation wave of one attenuation, its initiative oscillation peak value decides to be worth at the Q of oscillating circuit: Q value is greater, peak value is greater. Q value is little, criterion peak value is little. To reduce peak value, can reduce the leakage feeling Lep of transformer, increase the impedance R of Cds and circuit. And joining Snubber circuit is extremely effective method.

 

 

Oscillation 2 happen in Mosfet Q1 to close, deputy edge diode by change direction close, inductance of former edge excitation is released (at this moment Cds is filled to come 2Vc1) , cds and the stray capacitance Clp that primary foul line encircles are shunt-wound condition, mix again Lp of former edge inductance (excitation inductance and leakage feeling the sum) produce vibration. Discharge loop is the same as oscillation 1. Oscillation frequency is:

 

 

The alternate of oscillation voltage Vlp on Lp is added on Vc1, cause Vds=Vc1+Vlp. Measure Lp=0.4mH; Q1 is 2611, check norms book to be able to get Coss=190pF (Coss is equal to Cds approximately) , and this charges board for two pipe paralell connection, so Cds=380pF; Clp is measured so that be Clp=1.6nF when 200KHz. By on type can get: F =178.6KHz, with be identical of the 190.5K in next graphs.

 

 

Our exercisable improves ● measure has two:

 

★ 1, the size that reduces Noise;

★ 2, cut off or improve transmission way.

 

1. reduces the size of Noise:

 

Consider above all the following 3 respects:

 

When movement of ① Mosfet, Diode, have very wide spectrum content, the harmonic itself of switch frequency is stronger interference source.

 

Measure: Below condition of the efficiency that asks in contented place, temperature rise, we can pick the pipe with gentlier switch as far as possible. And through adjusting drive resistor also can achieve this one goal.

 

 

1 meeting produces the vibration of ② Q1, D1 stronger interference.

 

Measure:

 

 The * processing to parasitism capacitance Cds, Cj: In Q1 the two end of Ds pole, diode each go up one 681 small electric capacity, will reduce the Q cost of circuit, reduce the amplitude A of oscillation thereby, can reduce oscillation frequency F at the same time. Those who require an attention is: 1/2Cu2 will use up the energy of this capacitance to go up in Q1 entirely, so pipe temperature rise is a problem. Settlement way is use RC Snubber, let energy use up go up in R. At the same time R can rise to reduce the effect of amplitude.

 

The * processing to the leakage feeling Le of transformer:

 

1, transformer uses sandwich to wind a way, in order to reduce leakage feeling.

2, add on the winding of transformer draw circuit.

3, reduce Q1 D pole to arrive the down-lead length of transformer. (this down-lead inductance and change of leakage feeling posture are added) adopt afore-mentioned measure to reduce vibration the plan must lay after the influence of 1.

 

 

③ : 2 meetings generation disturbs the oscillation on Q1 D1 more by force.

 

Analytic method and ② are identical, but right now inductance already became very big (basically be for excitation inductance) , because this leaks,feeling and down-lead inductance are opposite the influence of ③ is relatively minor.

 

Same from inside the analysis above, the transmission way that can see Nosie basically is the stray capacitance Ctx that passes transformer;
 

Mosfet/Diode arrives the stray capacitance Cm/Cd of carbon fin; Reach carbon fin to the way such as the stray capacitance Ce of the ground coupling arrives LISN by capture of sampling resistor place.

 

 

Measure one: The ground of the ground end in Rs and C2 is indirect capacitance of a Y (472) .

 

Principle analysis: Its action is double, it is to be Mosfet movement to arise and the Noise electric current that strings together transformer deputy edge (be like I4) , provide the return of a low impedance, reduce the electric current of the ground. 2 it is to be what side Diode arises 2 times and the Noise electric current that strings together transformer former edge provides low impedance return, reduce the electric current that has shed LISN thereby.

 

Its effect pursues as follows: Red is: Before was not being improved; Blue is: After taking step

 

 

Measure 2: Transformer adds Faraday copper annulus:

 

Transformer is one of main thoroughfares that Noise transmits, among them primary coil and Ctx of capacitance of the stray between secondary coil are main factor. And Faraday copper annulus is being added in transformer interior is one of effective methods that reduce Ctx.

 

 

 

 

 

Measure 3: Carbon fin receives the ground end of Rs:

 

 

Purpose for carbon fin – Ce— ground – bypass of this one spur track drops LISN, reduce the electric current of the ground thereby. Its effect pursues as follows: Can see, more effective when low frequency; Be in high frequency when, the effect is not apparent, because be in,this basically is high frequency when, pin already had the effect that comparative to the capacitance of the ground directly.

 

Red is: Carbon fin not ground connection; Blue is: Carbon fin ground connection

 

 

When integrated after afore-mentioned all measure, if plan institute shows,EMI total effect contrasts:

 

Red is: Before taking step; Blue is: Integrated after afore-mentioned measure

 

 

Brief introduction of international attestation system

 

● Europe area:

 

Attestation EMC Mark EMC

Standard Cent disturbs a test for EMI (electromagnetism)&Photograph of EMS (electromagnetism allows a test) two parts:

 

★ 1. EMI part is EN55022, EN61000-3-2, EN61000-3-3;

★ 2. EMS part contains 7 tests for EN55024 :

 

EN55022 is Radiation Test&Conduction Test (is conducted&Shoot a test) ;

EN61000-3-2 checks for harmonic of power source of Harmonic Test () ;

EN61000-3-3 changes for voltage of Flicker Test (test)

EN61000-4-2 is electrostatic test of ESD Test () ;

EN61000-4-3 is electron of EFT Test (for RS Test EN61000-4-4 Xiao of fast arteries and veins checks) ;

EN61000-4-5 checks for be struck by lightning of Surge Test ()

EN61000-4-6 is conducted for CS Test (be able to bear or endure get degree of test) ;

EN61000-4-8 checks for light of voltage of DIP Test (for PFMF Test EN61000-4-11)

 

● America area:

 

Attestation EEMI Mark FCC (is mandatory)

Electromagnetism of Standard FCC Part 15 (EMI disturbs a test)

Application means

 

1.Attestation of Class A ego

2.Means of attestation of ego of Class B DOC

3.Attestation of TCB of Class B by, obtain FCC ID Number

 

< is noted>ITE product can send FCC attestation directly previously, obtain FCC ID, but remove FCC from 2000/11/15 already no longer attestation, accredit sends ID Number by TCB, and TCB accepts American place lab place to send a case only, intention is encouraging other area with DOC means attestation.

 

< is noted>Alleged ego attestation, test Report is issued after lab test finishs by qualification namely, autograph is believed at declare after application company obtains a report can.