Any issue with copper EMC shield over 1mhz DCDC inductors? - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2020, 10:49:06 am
content: I had two of my charger prototypes blow up. Both failures occurred after I had soldered a copper shield over the DCDC section for EMC testing. I'm positive I did not short anything to the copper.I was also doing other things at the time so the reason for the failures may not have anything to do with the copper can.However it made me wonder if putting a copper can 2mm above an inductor being switched at 1Mhzmay have thrown off the tuning. Or acted like a shorted turn. Or caused large HV spikes that killed it.Does that at all seem possible?The inductor I'm using is one of theses, so it's open at the top.I can do more testing to figure this out once more ICs arrive, I only had a few and now they're dead. hehe

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 10, 2020, 11:13:01 am
content: That does sound unlikely but.....In your picture it has SMT terminations at the sides, are you sure you did not short them ?It might just be the thing has a reliability problem anyway and the presence of the can was co-incidentalAhh did you connect your shield to something ? I always let mine float.I frequently use shields of this kind around ETD cores, outside the outer legs so they are not within the magnetic circuit completely successfully.EDIT thinking about that picture again is that circular "hole" a gap in the magnetic circuit ?If so high eddy currents will flow in any conductor in it's vicinity and cause all sorts of upsets.If it is a gap try leaving it either uncovered or ensure the shield is spaced well away from it (maybe 10-20mm).

author: KT88
author_rank: Frequent Contributor
timestamp: December 10, 2020, 12:31:54 pm
content: These magnetically shielded inductors have strong magetic field close to the case. The strenght rolls off very quickly with distance. This way you create almost a short by putting a copper layer across it. The effect is indeed caused by eddy currents...If you are experiencing radiated emissions coming for the inductor - have you checked the polarity of it? I can't identify the manufacturer to check it myself but I can tell that the package shows a marking. You always want to have the switching node connected to the winding start (the inner part of the winding). That way you get sort of a coaxial arrangement.CheersAndreas

author: jonpaul
author_rank: Super Contributor
timestamp: December 10, 2020, 12:48:41 pm
content: Any open airgap inductor creates an external field, and will have L and losses, R changed by presence of any conductor.You should find a closed mag ckt inductor to replace the one pictured.Reccomend CoilCraft catalog.Toroids and EE, EI or powdered iron toroids may be a solution.Jon

author: Psi
author_rank: Super Contributor
timestamp: December 10, 2020, 11:33:03 pm
content: Thanks! Lots of good info.The metal can was grounded.The reason for adding it was because i was seeing lots of 443mhz noise.However i suspect that was due to some wires i had attached to the chips NTC temp pin at the time.Because i retested and couldnt see any noise and the only change was no wires out to an ext pot i was previously using to simulate temp.Will switch to a sealed toroid inductor anyway

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 10, 2020, 11:51:58 pm
content: As on page 9 of this...file:///C:/Users/andrew/AppData/Local/Temp/an18.pdfalso here...https://ac-dc.power.com/design-support/product-documents/application-notes/an-18-topswitch-flyback-transformer/Also pg20 of thishttps://ac-dc.power.com/design-support/product-documents/application-notes/an-15-topswitch-power-supply-design/...a "closed loop" strip of copper tape is often stuck right round a switch mode transformer.....right over the turns themselves.So i wonder if you actually shorted those pads that we see in your picture.

author: Psi
author_rank: Super Contributor
timestamp: December 11, 2020, 11:38:29 pm
content: I was very careful to make sure the metal can didnt touch the inductor. Im sure it wasnt that

author: ElectronRob
author_rank: Contributor
timestamp: December 13, 2020, 05:16:15 pm
content: A quick and dirty testDrive your inductor with a signal generator w/parallel cap, adjust the frequency until you find peak.Then move your copper shield closer to the inductor and watch what happens. If the amplitude changes (likely goes down) then you know your EMC is having an effect here. 2min test.Rob