Disastrous (EMC) copper pour connected to chassis? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 19, 2022, 08:42:11 pm
content: Hi,We are doing 300W isolated Synchronous Pushpull, 24vin, 32vout, 225khz. PCB screwed in bottom of Alu enclosure. FETs are surface mount. 4 layer PCB. No daughter boards.The attached shows the layer 2. All of that copper pour (yellow) is the same net as chassis.You can also see where the planar transformer goes (the blue bit). You can also see the 6 screwholes by which the PCB screws into the Alu enclosure.There 6 Y caps…2 Y caps connect input ground to chassis connected coppe2 Y caps connect input power to chassis connected copper2 Y caps connect output ground to chassis connected copperThere is an output common mode choke but no input common mode choke.Question…..1…Why has a whole PCB layer been devoted to chassis?…..it is directly over the bottom mounted FETs…..and so the FET switching nodes will be pumping common mode noise into the shown chassis copper pour. Do you agree this chassis copper pour should not be there?

author: Whales
author_rank: Super Contributor
timestamp: November 19, 2022, 09:10:32 pm
content: Quote from: Faringdon on November 19, 2022, 08:42:11 pmSo chassis is not the same as the high power/current grounds? They are only tied in a few locations by Y caps?Is there a power ground layer behind the "chassis ground" layer?If not then some currents will probably be induced in this outer ground layer. They will have to take higher impedance paths (compared to a power ground layer), as they have to find one of the caps to get back to the rest of the circuit. This would probably increase EM emissions.(Did the designer have a reason to do it this way? Previous board performance issues?)

author: nctnico
author_rank: Super Contributor
timestamp: November 19, 2022, 10:07:14 pm
content: Every EMC guru will tell you to connect the chassis to the ground of your circuit. In this case the ground (0V of the circuit) isn't connected to chassis so I'd only connect to the chassis at the places where the Y capacitors need to connect to the chassis (ground). And I also see traces with very little clearance a split in this chassis ground plane. That is a red flag. All in all I'd get rid of this layer and re-think the grounding strategy. A good option would be to let all the Y caps meet in the same place and connect to the chassis at that point. Then currents flowing between primary to secondary can flow there and can not be tempted to take a shortcut through the chassis.If the ground of the input & output can be connected together, then connect them together AND to the chassis. Having these seperated causes more problems then it solves.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2022, 11:08:25 pm
content: "Isolated" but it's only like 100V?"Chassis" is plastic enclosure right? Surely the screw holes aren't isolated mechanically.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: November 19, 2022, 11:28:01 pm
content: QuoteThanks, sorry, no , the PCB is totally enclosed in a metal (alu) box.QuoteYes, requirement requests 1500V isolation pri to secQuoteThanks..You are right.....chassis is not connected to circuit ground, not to primary ground, and not to secondary ground....or rather , it only connects to them via the y caps.QuoteYes there is...the chassis copper pour (shown yellow in top post), is layer 2......layer 3 is circuit ground......so half primary ground, and half secondary ground.QuoteI think the designer was in a heck of a rush.....he did a lousy feedback circuit involving a secondary side error amplifier feeding into a digital isolator....and the digi isolator output connected to the comp pin of the primary side LTC3723-1 controller.QuoteI am not aware of any previous board.___---___--___---___---___--The following document...https://www.vicorpower.com/documents/design_guides/DG-DCM-Design-Guide-VICOR.pdf...on pages 86-96, shows the chassis copper being used as a copperpour right under a SMPS Power module.(several 100 Watts).They give no rationale for this.It appears to be madness, as the common mode noise from the switching stage will just get coupled into the chassis more easily, via this plane (the chassis plane is shown in yellow).....which is precisely what is not wanted.....all common mode noise filtration is designed around stopping noise capacitively and radiatively coupling to the chassis.So do you know why they do it like this?___---___---____--______As you know, there is a very simple rule in common mode filtration for EMC......"keep switching nodes as far away from chassis as you possibly can".This rule always applys.This rule has clearly been violated in the extreme by the copper pour connected directly to chassis in the top post...that copper pour is under the switching FETs.....well....i have to confess, there is a circuit ground layer shielding between this chassis copper pour and the switching FETs...so some would argue that the switching nodes are shielded from the chassis copper pour......But thats like saying you avoided getting hit by a train by moving off the tracks whenever a train came...i mean...if you're not a rail worker, why be on the tracks in the first place, ever?I mean...there is utterly no reason to have the chassis connected copper pour on the layer 2 as in the top post...would you agree?

author: coppercone2
author_rank: Super Contributor
timestamp: November 20, 2022, 12:51:55 am
content: I recommend testing it rather then fucking with it

author: nctnico
author_rank: Super Contributor
timestamp: November 20, 2022, 12:59:20 am
content: It will likely fail conducted and emissions the way it is. Input filtering will be necessary. How much can only be determined by doing pre-compliance measurements.

author: Whales
author_rank: Super Contributor
timestamp: November 20, 2022, 11:31:08 am
content: Quote from: Faringdon on November 19, 2022, 11:28:01 pmSo the entire board is already shielded by a combination of primary power ground and secondary power ground, with the chassis ground acting as a second unimportant layer?

author: Faringdon
author_rank: Super Contributor
timestamp: November 20, 2022, 12:53:52 pm
content: QuoteThanks, that sounds like it , yes.When you say "unimportant" layer, i assume you refer to the fact that its not performing any useful function?...so you believe that the layer of (sec and pri) ground on layer 3 actually shields the switching nodes from this chassis copper pour (which is on layer 2), so it wont be a problem?I would actually prefer to have this layer 2 as another (sec and pri) ground copper pour.....or use it for tracking.

author: Whales
author_rank: Super Contributor
timestamp: November 21, 2022, 06:00:44 am
content: Quote from: Faringdon on November 20, 2022, 12:53:52 pmOn the contrary, it might have a positive impact. Think of the chassis earth layer as performing a similar function to an external shield (such as an aluminium box) around your PCB.EMC is complicated. I cannot possibly claim the board will be better or worse if you replace the chassis earth layer with something else, I would be guessing. This isn't a simple comparison situation (like comparing having no ground planes vs some ground plane), you would have to build the devices and do actual testing to find out what happens.If you do tests then you might get surprising results, like emissions of your board being abysmal regardless of the ground plane configuration because of (eg) conducted emissions through your wiring harnesses. The ground plane design variations might be an unimportant factor, only changing a small % of overall performance.

author: Faringdon
author_rank: Super Contributor
timestamp: November 21, 2022, 06:58:46 am
content: QuoteThanks, thats a good point....in fact, this PCB is inside a totally enclosing aluminium enclosure.Regarding the shielding, i would think that a layer of ground copper would have been a better shield.AYK, regarding common mode EMC emissions, the golden rule is, "thou shalt avoid coupling circuit noise into the chassis".......this is really, "thou shalt avoid coupling circuit noise into the surroundings"....however, the chassis is the outer part of the system, so is the medium via which the circuit can couple noise into the surroundings.So given the above rule......it surely makes no sense to have a whole poured copper layer of the chassis net.?

author: uer166
author_rank: Super Contributor
timestamp: November 21, 2022, 07:41:40 am
content: Quote from: Faringdon on November 21, 2022, 06:58:46 amWhat if chassis is tied to your circuit GND and incoming mains as well.. There's nothing wrong with having the circuit GND reference be tied to chassis in multiple points, it just becomes one big reference net without any distinction. You do need to design for it.

author: ConKbot
author_rank: Super Contributor
timestamp: November 21, 2022, 05:05:49 pm
content: CHS plane split in two and both connected to the chassis via screws so common mode currents flow though the chassis instead of though class Y caps? That's quite the loop area. Also 1500v isolation and teeny clearance from the transformer pads to chassis?

author: Simon
author_rank: Unknown
timestamp: November 21, 2022, 06:26:22 pm
content: I'm completely confused, is this yours or something you are copying? it looks like both sides are connected to chassis. I have just been solving some EMC issues by not connecting every corner of the board to the chassis as it was a conductor and massive 10dB transmitter aerial.

author: Faringdon
author_rank: Super Contributor
timestamp: November 23, 2022, 05:24:52 pm
content: QuoteCistomer sent it in as their engineer left and it doesnt work, because they messed up the feedback loop and lots of other things.Quote...Yes, glad you noticed it.....as i thought id gone on one...seeing the split in the chassis plane...and then seeing that both halves are connected to the same chassis (ie "Primary" and "secondary" halves are actually connected together......not the entire chassis copper pour should even be there anyway.

author: Simon
author_rank: Unknown
timestamp: November 23, 2022, 08:37:35 pm
content: There are times when you may want a chassis plane. But the only time I did that was when I has a board that was EMC gasketed to the chassis with the bottom copper layer acting as a chassis to separate inner and outer part of a control box. It passed military EMC.