EMC & con­ducted emissions

Elec­tro­mag­netic com­pat­i­bil­ity is abil­ity of an equip­ment or sys­tem to func­tion sat­is­fac­to­rily in its elec­tro­mag­netic envi­ron­ment with­out intro­duc­ing intol­er­a­ble elec­tro­mag­netic dis­tur­bances to other equip­ment, sys­tem or ser­vices in that envi­ron­ment. Elec­tro­mag­netic com­pat­i­bil­ity is an inter­ac­tion of elec­tri­cal and elec­tronic equip­ment with elec­tro­mag­netic envi­ron­ment and with other equip­ment, or elec­tro­mag­netic envi­ron­ment inter­ac­tion with elec­tri­cal and elec­tronic equip­ment.

Equip­ment and sys­tem is elec­tro­mag­net­i­cally com­pat­i­ble with its envi­ron­ment and other equip­ment and sys­tems, if it sat­is­fies the fol­low­ing three cri­te­ria:
1. It does not cause inter­fer­ence with other equip­ment and sys­tems;
2. It is not sus­cep­ti­ble to emis­sions from other equip­ment, sys­tems and envi­ron­ment;
3. It does not cause inter­fer­ence with itself.

Design­ing elec­tro­mag­netic com­pat­i­bil­ity is not only impor­tant for the desired func­tional per­for­mance. Equip­ment and sys­tems must also meet legal require­ments before they can be offered in the mar­ket. There are no har­mo­nized elec­tro­mag­netic require­ments all over the world. Require­ments usu­ally vary from coun­try to coun­try. Although, there are coun­tries which agreed to har­mo­nize the require­ments to enable the free move­ment of goods, with­out coun­try to coun­try spe­cific require­ments. As an exam­ple Euro­pean Union can be men­tioned; it has devel­oped har­mo­nized elec­tro­mag­netic com­pat­i­bil­ity stan­dards and issued Elec­tro­mag­netic com­pat­i­bil­ity direc­tive that should be adopted in all mem­ber states.

Basi­cally, elec­tro­mag­netic com­pat­i­bil­ity is con­cerned with the gen­er­a­tion, trans­mis­sion and recep­tion of elec­tro­mag­netic energy. These three aspects are illus­trated in Fig.1.1. Source gen­er­ates the emis­sion and a trans­fer or cou­pling path trans­fers the emis­sion energy to a recep­tor, where it is result­ing in either desired or unde­sired behav­ior. Elec­tro­mag­netic inter­fer­ence (EMI) occurs if the received energy causes the recep­tor to behave in an unde­sired man­ner.
Trans­fer of elec­tro­mag­netic energy occurs via cou­ple of paths. Usu­ally these paths are unin­ten­tional and elec­tronic engi­neers are unaware of them. Paths rep­re­sented in Fig. 1.1. can be observed as paths between two devices or sys­tems as well as paths between com­po­nents on PCB.

These paths are:
1) Con­duc­tive cou­pling;
2) Induc­tive cou­pling;
3) Capac­i­tive cou­pling;
4) Elec­tro­mag­netic cou­pling.

Three aspects of electromagnetic compatibility- source, path, receptor

Fig. 1.1. Three aspects of elec­tro­mag­netic com­pat­i­bil­ity– source, path, recep­tor

Con­duc­tive cou­pling hap­pens when the cou­pling path between the source and the recep­tor is formed by direct con­tact with a con­duct­ing body. Con­duct­ing body can be formed out of trans­mis­sion lines, PCB trace, wires, cables, heat sinks or enclo­sures. Induc­tive cou­pling hap­pens when a vary­ing mag­netic field exists between two con­duc­tors in a close dis­tance, induc­ing a cur­rent in nearby con­duc­tor. Capac­i­tive cou­pling hap­pens when a vary­ing elec­tri­cal field exists between two adja­cent con­duc­tors in a close dis­tance, induc­ing a change across the gap between these two con­duc­tors. Elec­tro­mag­netic cou­pling hap­pens when source and recep­tor are sep­a­rated by a large dis­tance, source emits elec­tro­mag­netic energy and recep­tor receives elec­tro­mag­netic energy in terms of elec­tro­mag­netic waves. It should be noted that “close dis­tance” in EMC usu­ally is defined as dis­tance where mag­netic field is dom­i­nant and field strength varies as 1/​r3, 1/​r2, where r is dis­tance from radi­a­tion source. “Large dis­tance” is a dis­tance in which the field strength varies as 1/​r2 and wave imped­ance depends on medium it is prop­a­gat­ing through (e.g. 377 Ω in vac­uum).

To pre­vent elec­tro­mag­netic inter­fer­ence three options exist:
  • Decrease emis­sions at the source;
  • Decrease cou­pling path efficiency;
  • Increase recep­tor susceptibility.

The most effi­cient and eco­nom­i­cal way to reduce elec­tro­mag­netic inter­fer­ence is to decrease the emis­sions at the source. This requires highly qual­i­fied engi­neers to mod­ify the cir­cuits to decrease emis­sions, but still pro­vide the nec­es­sary func­tions. Recep­tor sus­cep­ti­bil­ity improve­ment also requires highly qual­i­fied engi­neers to mod­ify cir­cuits to han­dle high dis­tur­bance impact and still pro­vide its func­tions. Cou­pling path effi­ciency reduc­tion is not as advanced as two tasks men­tioned above. There is no need for source and recep­tor inter­nal cir­cuit man­age­ment. It is only cru­cial to know the dis­tur­bance prop­a­ga­tion path. Var­i­ous tech­niques to decrease path effi­ciency exist, but one tech­nique is com­mon for all paths reduc­tion. It is a fil­ter appli­ca­tion. In Fig. 1.2. the appli­ca­tion of fil­ters is explained using cou­pling paths defined in Fig. 1.1.

Filter application to reduce coupling paths between source and receptor

Fig. 1.2. Fil­ter appli­ca­tion to reduce cou­pling paths between source and recep­tor

Fil­ter installed at the input and out­put cables of devices and sys­tems reduce the con­ducted dis­tur­bance path effi­ciency. If the fil­ter is installed in the right posi­tion and cor­rectly wired, con­duc­tive dis­tur­bance reduc­tion will lead also to induc­tive and capac­i­tive noise reduc­tion. There­fore, elim­i­na­tion of one dis­tur­bance path in the right region will lead to elec­tro­mag­netic inter­fer­ence issue elim­i­na­tion. If the con­ducted dis­tur­bance is reduced at the right region, elec­tro­mag­netic dis­tur­bances are reduced and there should be no fur­ther prob­lems regard­ing elec­tro­mag­netic inter­fer­ence.

Since unwanted dis­tur­bances usu­ally are at much higher fre­quen­cies than use­ful sig­nals (50Hz mains power, com­mu­ni­ca­tion sig­nals, sen­sor sig­nals etc.) fil­ter works by selec­tively block­ing or atten­u­at­ing unwanted higher fre­quen­cies. Basi­cally, the induc­tive part of the fil­ter is designed to act as a low pass device to enable low fre­quency use­ful sig­nal trans­mis­sion thru the line and atten­u­at­ing high fre­quency sig­nal com­po­nents. Other parts of the fil­ter use capac­i­tors to bypass or shunt unwanted high fre­quency dis­tur­bance. There­fore, pas­sive elec­tro­mag­netic inter­fer­ence fil­ters are a com­bi­na­tion of induc­tors and capac­i­tors, where each com­po­nent has its pur­pose. Unwanted dis­tur­bance sig­nal can also appear on antenna ter­mi­nals, thus feed­ing unwanted sig­nals to antenna, in com­bi­na­tion with use­ful sig­nals. In his case band pass fil­ters are applic­a­ble– fil­ter is designed to pass the use­ful sig­nal, but atten­u­ate unwanted sig­nal com­po­nents.

In gen­eral EMI prob­lems that must be solved by fil­ter appli­ca­tion (or other method usage) are illus­trated in Fig. 1.3.:
  • Con­ducted emissions;
  • Radi­ated emissions;
  • Con­ducted susceptibility;
  • Radi­ated susceptibility.
There are huge amount of EMC stan­dards that limit the elec­tronic prod­uct con­ducted emis­sions. These stan­dards can be divided in var­i­ous groups, depend­ing on envi­ron­ment, prod­uct group or spe­cific prod­ucts. Require­ments in these stan­dards dif­fer from prod­uct to prod­uct, depend­ing on elec­tronic prod­uct power, typ­i­cal usage envi­ron­ment, etc. One of the most widely spread stan­dard is CISPR 11 or its ana­log EN 55011 “Indus­trial, sci­en­tific and med­ical equip­ment– Radio-​frequency dis­tur­bance char­ac­ter­is­tics. Lim­its and meth­ods of mea­sure­ment.”.

Electromagnetic compatibility emission and susceptibility classification

Fig. 1.3. Elec­tro­mag­netic com­pat­i­bil­ity emis­sion and sus­cep­ti­bil­ity clas­si­fi­ca­tion

It defines the con­ducted emis­sion mea­sure­ment method­ol­ogy and defines the con­ducted emis­sion lim­its. In Fig. 1.4. EN 55011 con­ducted emis­sion limit lines are pre­sented for class A and class B equip­ment.

EN 55011 conducted emission limit lines

Fig. 1.4. EN 55011 con­ducted emis­sion limit lines

Con­ducted emis­sion require­ments are defined in range 150kHz– 30MHz. The mea­sure­ment method­ol­ogy is defined in Fig. 1.5. Line imped­ance sta­bi­liza­tion net­work (LISN) is used to mea­sure dis­tur­bances cre­ated by elec­tronic prod­ucts or equip­ment under test (EUT). EUT is con­nected to the mains net­work through LISN due to fol­low­ing rea­sons:
  • LISN defines mains impedance;
  • LISN fil­ters acts as high pass fil­ter and inter­face to EMI analyzer;
  • LISN acts as fil­ter and atten­u­ates noise com­ing from mains network.

 Conducted emission measurements according to EN 55011

Fig. 1.5. Con­ducted emis­sion mea­sure­ments accord­ing to EN 55011

LISN imped­ance and con­struc­tion para­me­ters are defined by stan­dards. Sim­pli­fied LISN schematic is pre­sented in Fig. 1.6. LISN actu­ally is a high pass fil­ter. 50Hz power fre­quency is trans­mit­ted through LISN power ports with­out any atten­u­a­tion from side of mains net­work, but high fre­quency com­po­nents are for­warded to high fre­quency out­put (50Hz com­po­nent is not present at high fre­quency out­put). Dis­tur­bances from EUT are ter­mi­nated in defined imped­ance by the LISN (at 30MHz line-​ground imped­ance is 50) and mea­sured by EMI receiver.

LISN internal circuit

Fig. 1.6. LISN inter­nal cir­cuit

To mea­sure con­ducted emis­sions from one phase equip­ment, it is nec­es­sary to have LISN con­tain­ing com­bi­na­tion of two cir­cuits defined in Fig. 1.6., one for line wire and sec­ond for neu­tral wire. For three phase EUT emis­sion mea­sure­ments there should be LISN with com­bi­na­tion of four cir­cuits defined in Fig. 1.6. One phase EUT con­nected to one phase LISN is shown in Fig. 1.7.

Con­ducted dis­tur­bances are flow­ing in all wires con­nected to EUT– line, neu­tral and ground­ing. To sim­plify dis­tur­bance analy­sis dis­tur­bances are divided in two groups:
  • Com­mon mode (CM) emissions;
  • Dif­fer­en­tial mode (DM) emissions.

Dif­fer­en­tial and com­mon mode cur­rents are shown on sim­ple case in Fig. 1.7. Imped­ances Z1 and Z2 rep­re­sent the line imped­ance of EUT and Z3 rep­re­sents EUT imped­ance to ground. Dif­fer­en­tial mode cur­rent iDM flows into line-​neutral loop. Com­mon mode cur­rent iCM flows in ground wire and cre­ates two loops though line and neu­tral wires. There­fore, cur­rent in each wire sep­a­rately can be cal­cu­lated:
i1=iDM–iCM, (1.1)
i2=-iDM–iCM (1.2)
i3=2iCM (1.3)

Volt­age mea­sured by LISN is (accord­ing to EN 55011 Line-​Ground and Neutral-​Ground imped­ance is 50):
v1=50(iDM–iCM), (1.4)
v2=50(-iDM–iCM). (1.5)

Accord­ing to equa­tions (1.4) and (1.5), LISN is mea­sur­ing volt­age drop cre­ated either by addi­tion or sub­trac­tion of com­mon and dif­fer­en­tial mode cur­rents. To be in com­pli­ance with EMC stan­dards these volt­age drop val­ues should be lower than limit line defined val­ues.

One phase equipment conducted differential mode and common mode emission measurements

Fig. 1.7. One phase equip­ment con­ducted dif­fer­en­tial mode and com­mon mode emis­sion mea­sure­ments

The pur­pose of the EMI fil­ter is to reduce volt­age drop V1 and V2 to com­ply with EMC stan­dards. Dif­fer­en­tial mode and com­mon mode cur­rent mag­ni­tude depends on dis­tur­bance source– EUT. There could be a sit­u­a­tion that one of dis­tur­bance mode dom­i­nates over the other one. There­fore, there is no uni­ver­sal fil­ter cir­cuit that guar­an­tees the com­pli­ance with EMC stan­dards. In each sit­u­a­tion fil­ter inter­nal cir­cuit should be engi­neered to effec­tively reduce dis­tur­bance emis­sions.

The basic prin­ci­ple of com­po­nents in EMI fil­ter is to bypass dis­tur­bance source using shunt capac­i­tors and intro­duce high imped­ance to dis­tur­bance using series induc­tors. Fil­ter can be made out of sin­gle com­po­nent– first order fil­ters– such as induc­tor or capac­i­tor or more advanced topolo­gies cre­at­ing sec­ond, third etc. order fil­ters intro­duc­ing cou­ple of induc­tors and capac­i­tors. Each dis­tur­bance mode has its ded­i­cated com­po­nents and fil­ter topolo­gies, that are used to reduce the cur­rent dis­tur­bance mode emis­sions.

Dif­fer­en­tial mode cur­rent is flow­ing thru two wires (line-​neutral in Fig. 1.8.), there­fore DM fil­ter is con­structed and con­nected only to these two wires. Basi­cally, DM fil­ter con­sists of DM induc­tor and x-​capacitor cre­at­ing LC fil­ters that can be con­nected in cas­cades. In Fig. 1.8. DM fil­ter is intro­duced, con­tain­ing two LC cas­cades. Induc­tor L2 cre­ates high imped­ance for high fre­quency dis­tur­bances and capac­i­tor CX2 is bypass­ing it. L1 and CX1 are accom­plish­ing the same task, result­ing in decreased volt­age mea­sured by LISNV1 and V2. DM fil­ter has no impact on CM dis­tur­bance cur­rent.

Differential mode filter- two LC cascades

Fig. 1.8. Dif­fer­en­tial mode fil­ter– two LC cas­cades

Com­mon mode dis­tur­bance cur­rent is flow­ing thru all wires con­nected to EUT, there­fore CM fil­ter should be con­structed to reduce cur­rent com­po­nents in all wires. Basi­cally, CM fil­ter is cre­ated using CM chokes and y-​capacitors. In Fig. 1.9. CM fil­ter two LC cas­cade topol­ogy is intro­duced. Induc­tor L2 cre­ates high imped­ance for high fre­quency CM cur­rent, while capac­i­tors CY3 and CY4 are bypass­ing the CM cur­rent. The same task is accom­plished by CM choke L1 and capac­i­tors CY1 and CY2. It should be noted that y-​capacitors have impact on DM dis­tur­bance cur­rent, as CY1, CY2 and CY3, CY4 are con­nected in series and con­nected between line and neu­tral. A pos­si­bil­ity to con­nect induc­tor in series with ground wire also exists, but this kind of induc­tor appli­ca­tion has draw­backs regard­ing elec­tri­cal safety, due to lim­ited ground cur­rent capa­bil­ity and raised ground­ing imped­ance.

Common mode filter- two LC cascades

Fig. 1.9. Com­mon mode fil­ter– two LC cas­cades

To reduce DM and CM dis­tur­bances it is nec­es­sary to cre­ate fil­ter con­tain­ing both upper men­tion fil­ter char­ac­ter­is­tics– DM and CM fil­ter­ing com­po­nents. Finally, fil­ter, even for one phase equip­ment, con­tain­ing only sin­gle stage LC fil­ter for each dis­tur­bance mode, con­sists of at least four com­po­nents. If dis­tur­bance level is higher, the com­plex­ity of fil­ter increases.

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