Differential mode inductors to combat radiated emissions in cable to DCDC SMPS? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 06, 2021, 03:51:45 pm
content: We all use Differential mode LC filters to reduce conducted emissions flowing along cables to DCDC converters, so that we can pass whatever conducted EMC standard applies.However, often for DCDC converters fed from a cable connected to an upstream SMPS that has passed all of its EMC emissions testing, there is no need for differential mode filter inductors in the input LC filter, because no conducted EMC standards apply to such a cable.…in this case, we just use an input filter capacitor bank, and forget about diff mode filter inductors.However, do you think that NOT having such diff mode filter inductors could mean having high frequency diff mode currents flowing in the “loop” of current of the cable, and thus radiated emissions may then emit from the cable?So basically, to summarise, in cable fed DCDC converters, do differential mode filter inductors play any part in reducing radiated emissions from the cable?(I ask because as you know, usually radiated emissions from cables are associated with common mode currents in the cable, not diff mode currents.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2021, 09:18:28 pm
content: Err, what cable? If it's good coax or well balanced twisted pair, it doesn't radiate anything. Entirely matters what happens at the ends, and poor quality cable plays a similar role (e.g. leaky coax like RG-58, or pairs that aren't well twisted or are too close to surroundings, thus breaking the symmetry).And what signal level? That determines required CM-DM isolation, and what filtering if any is applicable.Tim

author: penfold
author_rank: Frequent Contributor
timestamp: March 06, 2021, 09:58:53 pm
content: Quote from: treez on March 06, 2021, 03:51:45 pmWhy is this cable exempt? EMC applies to the system as a whole in its "end state"

author: ocset
author_rank: Super Contributor
timestamp: March 07, 2021, 09:38:23 pm
content: QuoteThanks, so are you advising that if an EMC compliant offline flyback say, then has a cable and a DCDC SMPS attached to it, (as a load) then it may well then become non compliant in its EMC?So in other words, when supplying a 20W dummy load in its enclosure , the offline flyback passes EMC...but then make the load a 20W DCDC SMPS at the end of a cable, and then that same Flyback, that had previously passed its EMC, may then fail its EMC?

author: penfold
author_rank: Frequent Contributor
timestamp: March 07, 2021, 10:25:03 pm
content: Quote from: treez on March 07, 2021, 09:38:23 pmIt depends which standard you're working to, some may grant you some exemption from including external power supplies as part of the certification (I just don't know which standards these may be, some consumer goods maybe?).The generalistic statement is that yes there are potentially some characteristics of your equipment which may cause an EMC problem when used with a previously certified SMPS and EMC applies to your "system" which includes said SMPS.In response to your original question, common mode noise will be more greatly affected by the layout and grounding structure than the filter caps themselves, it depends so much more on other characteristics of your system than simply the input caps, also whether or not DCDC is isolating or not

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2021, 11:22:08 pm
content: Not that SMPS module certs are very meaningful, as they typically test them with a dumb resistor on a plate, no grounding at all. So common mode emissions at low frequencies may be wildly out of whack when the enclosure or output is grounded.You're certifying the whole thing anyway, just work to that. No one cares what you have on a cable, at least until such a test arises (there are some that specify a cable clamp to monitor or inject current on cables between EUTs, that meet certain criteria). That's not going to have anything more to do with the power inlet emissions, for example, than anything else in the system. Analyze the system as a whole, if it's going to be a problem then don't do that, add the filter or shield or whatever is needed, and that's that.Tim