LED lumi­nary EMC

A stan­dard exam­ple of a power sup­ply cir­cuit for LED light­ing equip­ment, using con­ven­tional power com­po­nents is shown in fig­ure 1. This kind of con­struc­tion will lead to exces­sive radi­ated emis­sions. One of solu­tions is to shield entire hous­ing. Shielded enclo­sure increases the par­a­sitic capac­ity between the chas­sis and PCB and the ref­er­ence ground of any EMC mea­sure­ment equip­ment. Common-​mode con­ducted emis­sions then becomes a large enough phe­nom­e­non to exceed con­ducted emis­sion limit lines. In this case EMI fil­ter is added to elim­i­nate con­ducted emis­sions at the LED lumi­nary AC power input. Sim­i­lar sit­u­a­tion is at the DC out­put. LED diodes are placed on heat sink, lead­ing to com­mon mode cur­rent that is respon­si­ble of radi­ated emis­sions from wires or PCB traces.

The main prob­lem in LED light­ing equip­ment is the power supply’s high-​speed switch­ing cir­cuits, which cre­ate wide spec­trum cur­rent and volt­age rip­ples at the mains AC input and DC out­put. Proper shield­ing and fil­ter­ing can mit­i­gate the radi­ated and con­ducted emis­sion prob­lems, but do not elim­i­nate it. A bet­ter solu­tion would be to avoid gen­er­at­ing high radi­ated emis­sions and con­ducted emis­sions at par­tic­u­lar fre­quen­cies in the first place. This is pos­si­ble through the use of new power com­po­nents that use soft switch­ing to min­i­mize rip­ple cur­rents, or to spread the noise energy over a wide fre­quency band in prop­erly designed power sup­plies.

Fig­ure 1: Typ­i­cal AC/​DC LED dri­ver design. The H-​field is the result of wind­ing leak­age, the pri­mary loop area and the sec­ondary loop area. The E-​field is the result of high dV/​dt on con­duc­tive sur­faces and of high-​frequency rip­ple in cables.

The Power Fac­tor Con­troller (PFC) is the most com­mon block in mod­ern AC/​DC LED dri­vers. PFC is respon­si­ble for power fre­quency har­monic cur­rent emis­sions. This is one of the manda­tory EMC emis­sion tests, defined by EN 6100032. Accord­ing to EN 6100032 PFC is manda­tory if LED lumi­nary power exceeds 25W. In fig­ure 2 har­monic emis­sion results are pre­sented for LED light­ing equip­ment with and with­out PFC. In case of power fac­tor cor­rec­tion cur­rent drawn by the LED lumi­nary is close to sine wave, while with­out PFC cur­rent is drawn in peaks.

Fig­ure 2 Har­monic emis­sions accord­ing to EN 6100032

Con­ducted emis­sions are cre­ated by LED dri­ver and con­ducted via all cables con­nected to dri­ver. Con­ducted emis­sions for LED light­ing devices are lim­ited by inter­na­tional stan­dard EN 55015 or CISPR 15. Emis­sions should be mea­sured on all cables that are con­nected to aux­il­iary equip­ment– mains power, con­trollers, etc. Usu­ally there is only one port for lumi­nar­ies– that is AC mains con­nec­tion. Exces­sive emis­sions are lim­ited by EMI power input fil­ter. Proper fil­ter design will solve con­ducted emis­sion issue. In fig­ure 3 exam­ple of fail­ing con­ducted emis­sion results are pre­sented. In fig­ure 4 exam­ple con­ducted emis­sion results passes the EN 55015 require­ments. Con­ducted emis­sion mea­sure­ments are car­ried out by two EMI detec­tors– quasi-​peak (QP) and aver­age (AV). There­fore there are two limit lines on mea­sure­ment plots. Quasi-​peak detec­tor mea­sure­ment should be below QP limit line and aver­age detec­tor should be below AV limit line. Quasi peak limit line starts at 9kHz and quasi peak mea­sure­ments should be mea­sured start­ing at 9kHz, while aver­age detec­tor mea­sure­ments should be started at 150Hz.

Fig­ure 3 Con­ducted emis­sions accord­ing to EN 55015 (Fail)

Fig­ure 4 Con­ducted emis­sions accord­ing to EN 55015 (Fail)

Usu­ally first sweep is car­ried out by peak detec­tor (blue) instead of quasi-​peak detec­tor, as peak detec­tor mea­sure­ments are faster. After­wards, emis­sion peaks are re-​measured by quasi-​peak detec­tor– final mea­sure­ments (blue squares). Aver­age mea­sure­ments are cre­ated only with aver­age detec­tor.
Radi­ated emis­sions are cre­ated by high fre­quency cur­rent com­po­nents flow­ing in LED lumi­nary con­duc­tors, long enough to be an effi­cient antenna (AC power input cable, LED array and con­nec­tion wires, metal­lic enclo­sure etc.). Source of dis­tur­bances is LED dri­ver allow­ing high fre­quency cur­rent flow out­side the dri­ver enclo­sure. Radi­ated emis­sion mea­sure­ment result exam­ples are pre­sented in fig­ure 46. LED lumi­nary radi­ated emis­sions are mea­sured in fre­quency range 9kHz-​30MHz using mag­netic field antenna and 30MHz-​300MHz using log-​periodic or bicon­i­cal anten­nas. Mea­sure­ments are car­ried out using quasi-​peak detec­tor.

Fig­ure 5 Radi­ated emis­sions 9kHz-​30MHz accord­ing to EN 55015 (Pass)

Fig­ure 6 Radi­ated emis­sions 30-​300MHz accord­ing to EN 55015 (Pass)

Fig­ure 7 Radi­ated emis­sions 30-​300MHz EN 55015 (Fail)

EMC test­ing

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