Hi Muriel , group, What a filter tries to do is to create a short circuit routing for the EMI current back to the source. Therefore, filtering just the mains side to ground will annihilate it's radiation potential (relative to enclosure), but somewhere else potential will rise. This might very well be another cable leaving the enclosure. Therefore filtering the second cable will also become necessary in order to prevent "the balloon from popping out".
This is what happens all the time when reducing EMI by adding filters and other techniques. Only when the last exit port is filtered (for conducted), the EMI reduces significantly. It does not necessarily mean that the last port fixed had the highest contribution to the EMI; it was because it was the latest fixed. In this final situation we try to create a situation where the EMI current source is short circuited by an external circuit of caps/ground planes, and all cable exit points are of equal EMI potential: that of the enclosure ground plane. The EMI current source (or voltage source) is often placed in the GND circuit on a PCB where input and output are not essentially at the same ground potential due to RF current flowing into this ground trace; i.e. input and output are physically separated. Once the short circuit (filter - enclosure - filter) is created the energy will dissipate in the loop: EMI source - gnd trace - cable to filter - caps - enclosure gnd plane - caps of other filter - cable to PCB input - gnd trace on PCB - EMI source Voltage levels in general are low <10 mV, current may be relatively high (sometimes up to 100 mA). This is due to the low impedance of the loop described. By strategically placement of impedance this current may be reduced and the voltage drop over the outer surface of the enclosure may be minimized, at the cost of a higher voltage drop between sides of the PCB ground trace. This voltage is in general also partially present in series with both input and output signal (draw the loop above) and it may have a negative impact on signal integrity. Regards, Gert Gremmen, (Ing) ce-test, qualified testing =============================================== Web presence http://www.cetest.nl CE-shop http://www.cetest.nl/ce_shop.htm /-/ Compliance testing is our core business /-/ =============================================== >>-----Original Message----- >>From: owner-emc-p...@majordomo.ieee.org >>[mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Muriel >>Bittencourt de Liz >>Sent: Monday, December 03, 2001 10:00 PM >>To: EMC-PSTC List >>Subject: physics behind EMI powerline filter >> >> >> >> >>Hello Group, >> >>I have a "long-time" question, concerning the energy issues in a >>EMI powerline filter. >> >>I'll put a case, and ask the question after. >> >>This is the case: >> >>- When trying to minimize the conducted emissions from a >>electrical equipment / circuit, one of the things to do is to put >>a EMI filter at the power entrance. This filter can be from a >>manufacturer (ready filter) or you can make one (with common mode >>inductors, capacitors, inductors). >> >>This are the questions (they arearelated, i.e., complete each other): >> >>- What, physically speaking, happens to the EMI energy that >>leaves the equipment when I add a filter??? When there is no >>filter, I understand that the energy goes to the mains?? Does the >>filter reflect the EMI energy, keeping it "arrested" inside the >>equipment? >> >>- Thinking under the light of the principle of energy >>conservation, what happens to the EMI energy when I add a >>filter?? It cannot be lost... >> >>- ... And, supposing that the energy keeps arrested inside the >>equipment, isn't it worse for radiated emissions?? i.e., it can >>increase the level of radiated emissions?? >> >>Thanks in advance for your attention. >> >>Regards, >> >>Muriel Bittencourt de Liz >>Ph.D. Student >>Interest Areas: EMC for power electronics, RF measures, EM interference >>Federal University at Santa Catarina State >>Florianópolis, Santa Catarina, Brazil >> >> >> >>------------------------------------------- >>This message is from the IEEE EMC Society Product Safety >>Technical Committee emc-pstc discussion list. >> >>Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ >> >>To cancel your subscription, send mail to: >> majord...@ieee.org >>with the single line: >> unsubscribe emc-pstc >> >>For help, send mail to the list administrators: >> Michael Garretson: pstc_ad...@garretson.org >> Dave Heald davehe...@mediaone.net >> >>For policy questions, send mail to: >> Richard Nute: ri...@ieee.org >> Jim Bacher: j.bac...@ieee.org >> >>All emc-pstc postings are archived and searchable on the web at: >> No longer online until our new server is brought online and >>the old messages are imported into the new server. >> >>
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