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Conducted Emissions Fail (DC)

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AJBotha

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Hi Guys

I recently had a product that I designed tested at an EMC lab and at first it failed the conducted emissions test.

The PCB is very size constrained and I am looking for the most cost and size effective solution that cures the problem.

I replaced the polarity protection diode D1 with a 220uH inductor that I had on hand, and the PCB then passed,
but it was marginal, with the first spike almost exceeding the allowed value.

The issue was of course at the switching frequency, 500kHz. The first spike was at 500kHz and exceeded the limit.
The harmonics, all clearly vissible, was withing limit.

Should I leave it as it passed, should I implement a PI filter, or/and increase the inductance to increase the resistance to higher frequencies?

The plot is the plot of the modded PCB including the inductor, sorry for the bad quality, its the best I have atm.

Advice would be greatly appreciated! Thanks

**broken link removed**

**broken link removed**
 

Hi,

first: passed means passed. Therefore i see no need to change anything.
Every change means a new approval.

But I understand you want to improve on emissions.
One point is to place an additional capacitor in parallel to the transzorb. If this really improves the emissions is hard to say. I mainly depends on PCB layout.

A picture of your layout is helpful.
Also the wiring (power supply, load and any other cables) and the shield connections to case or cable if they exist.

Klaus
 

With SMPS's layout is critical, as Klaus has stated we would need to look at the layout....
 

There should actually be no wiring as the card plugs into a motherboard type of controller.
But for the test a 1m twisted cable was used from two batteries in series.
I focused on the layout near the switcher, didn't even think about conducted emissions! (First time noob here)

The connector is fixed, the pinout is fixed and available space is very little (to my standards at least).
The tranzorb is on the bottom, just off the two input pins, there is also a ground plane underneath the switcher.
(2 Layer PCB)

Temp2.JPG
 

Hi,

first: passed means passed. Therefore i see no need to change anything.
Every change means a new approval.

But I understand you want to improve on emissions.
One point is to place an additional capacitor in parallel to the transzorb. If this really improves the emissions is hard to say. I mainly depends on PCB layout.

A picture of your layout is helpful.
Also the wiring (power supply, load and any other cables) and the shield connections to case or cable if they exist.

Klaus

Hi Klaus

As far as I understand you can also "self-declare" that you are EMC compliant, right? I think in cases like this we try to only do BOM changes,
alternatively we only make very modest changes like changing to a footprint that accommodates the fix.

If we do update the PCB and not just retrofit the existing ones, I would like to have at least the conducted emissions re-tested,
I think there is much for me to learn especially in this area.

PS: When does this darn "your post needs to be approved" go away?
 

I worry about the layout, no real ground plane, SMPS is compromised, it would be many times better with 4 layers, most of your problems would go away.
I am not on my system at the moment but could you post either the SMPS circuit or the controller you are using please and I will gather some illustrations from my collection of hundred of SMPS layouts. My main concern is the main switching loops and especially the return/0Vs are not on one layer.
I would look at the costing's of going to a 4 layer board...
 

I have been trying to learn about SMPS EMI, from what i have been finding here is my advice.

Both of your power leads should have filtering and i believe that a ferrite chip will help you.

I doubt the transorb is doing anything for you unless you have very dirty power. I would replace it with a lower value capacitor (or possibly an RC) to shunt the 500khz. Pick the value of this capacitor experimentally. Connect an oscilloscope to the wires in question and monitor the waveform at 500khz and try different capacitors for lowest signal. Just replacing the transorb with the proper capacitor may be enough, but adding the ferrite chips should do a good job cleaning up the 500khz.

I picked out the most aggressive ferrite chip i could find but i bet you do not need the 880 ohms at 1 mhz that it has. Get an assortment of chips to try down to 200 ohms or so


DigiKey

Ferrite Beads and Chips

https://www.digikey.com/product-sea...e=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=500

NFZ32BW881HN10L
FERRITE CHIP 880OHM 200MA 1210
 

The tranzorb is there to dissipate any energy injected by surges, and also over-voltage protection.
This also sort of serves as polarity protection now that the polarity protection diode might have to be replaced.

Ferrite beads are mostly helpfull for high frequency signals as far as I know,
and a RC (Low pass) filter is not desired since you drop voltage over the series resistor.

The best bet to me looks like a PI filter.

Marce, unfortunately a 4 layer is not an option. I will post the proposed respin layout also, just looking around for some suitable parts.
 

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