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Please help with failed EMC scan for charity system?

Flyback

Well-Known Member
Hi
Please help our kiddies post-lockdown pager system game kit pass radiated EMC? I am doing this with a company in South Africa for charity. Its as attached.

It is an earthed metal enclosure A, which contains a 20W offline SMPS and a 12V battery. This enclosure feeds a 12V cable to enclosure B which contains a 10W , 12V to 5V SMPS. This 5V is then fed to each of four removable pagers, (so they get charged up) which the kiddies can remove and talk to each other. (they will be in a “hospitals and doctors” game)

It fails EMC at the moment. (Radiated emissions failure at frequencies from 80MHz to 190MHz) We wish to sell it into the EU, where they need some post-Covid relief as much as everyone else.

Should the cable between A and B be earth screened and the earth screen connected to both enclosure A and B?

The enclosure B is open at the front, so is it therefore a waste of time to earth enclosure B? As such, is it also a waste of time to take earthed 12V cable from A to B?

The wires running from the 12v_to_5v SMPS to each pager, should they be loomed as close as possible to the walls of enclosure B?

The enclosure B has to be open at the front so that kiddies can grab the pagers.

We are thinking we may need a common mode choke at the point in enclosure A where the 12V cable leaves A to go to B? Also, we could put a common mode choke at the input and output of the 12v_to_5v SMPS. (NiZn torroid based common mode chokes with just a few turns) We could also put a flux band round the offline flyback transformer in A. We could also increase the Y capacitor across the flyback transformer in A. At the moment its only 100pF. We don’t want PCB respins if poss as the customers don’t want to pay much.
 

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Flyback

Well-Known Member
It is well known that passing radiated emissions to EU standard with an SMPS that is not surrounded in a metal enclosure is pretty well impossible. Even if it is in a metal enclosure, it will still fail radiated emissions if a cable leaves the enclosure to go somewhere else. (as in the attached). In that case, a common mode choke and Y caps will be needed in order to stop the cable from radiating. Would you agree?
 

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kubeek

Well-Known Member
Most Helpful Member
waste of time is doing the emissions test, failling, and not trying the most simple thing of connecting the two boxes together with a piece of wire.
Cable between box A and B makes a really nice antenna for any noise coming from the SMPS.
Connect box A to B with ground wire, cable does not have to be shielded in my opinion. Use Y caps to tie the secondary of the smps to mains ground, and use caps to tie the low voltage ground in box B to its case to prevent any large potential difference between the secondary and the cases.
 

Flyback

Well-Known Member
Connect box A to B with ground wire, cable does not have to be shielded in my opinion. Use Y caps to tie the secondary of the smps to mains ground, and use caps to tie the low voltage ground in box B to its case to prevent any large potential difference between the secondary and the cases.
Thankyou, indeed we did all these things...and yes it made an improvement, but not enough.

I am thinking that we must use some shielding. Do you agree? Also, i believe we will need to re-do the DCDC SMPS with common mode chokes on its input and output...and have Y caps on the pcb to the pillars which screw into the earthed chassis B. Would you agree.?
Any better measures?
Also, Our cheapo radiated emissions test, is to torroid-wrap a near field B probe of diameter ~9cm with 18 turns of twisted pair wire (evenly spread round the loop)...........we then solder this twisted pair "wrap" into the cable between the two units A and B in order to gauge just how much radiation at each frequency is in the cable. (the B field probe ultimately gets BNC'd into an SA1002 spectrum analyser) Our problem frequencies are from 57MHz to 184MHz, with a highest peak at 96MHz.

I am wondering if another trick for us will be to take multiple different lengths of cable to the Radiated emissions lab, and keep swapping them in and out until we get a a pass..then we use that length on the product? (the cable between A and B)
 
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kubeek

Well-Known Member
Most Helpful Member
I would start with the smps, prepare a setup to test which of the power supplies is the problem. Measure the offline one first with a dummy resistor load at the end of the cable. If the first one is ok, then it has to be either your second psu or the load device itself. But since the only long cable is between A and B then I bet that cable is the antenna. Tie the output noise of A supply as best as possbile to be contained in box A, and tie input noise of B supply inside box B. Tie the DC lines as best as possible to PE before leaving box A.
 

Flyback

Well-Known Member
Thanks, Most of the noise is coming from the DCDC in box B.
and tie input noise of B supply inside box B
So do you mean with Y capacitors from DCDC 0V to chassis of B?
 

gophert

Well-Known Member
Most Helpful Member
Use a piece of flexible metal conduit to cover your 12VDC jumper connecting the two boxes.
 

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