TPS82130: EMC radiated emissions.

Hi Yuchang, we can't be 100% sure of the 30M peak origin, but we did some investigation with a sniffer probe (at the EMC test house) and that seemed to be where the radiation was coming from. These type of wideband humps at the lower end of the emissions test spectrum are usually power supply switching noise. In any case there is not at lot else going on the board that might cause this emission. At the test house I did try adding 100nF caps to the positions C8 and C43, with no success.

Does TI have experience of the TPS82130 passing class B emissions or similar issues?

My statement about the screening, is basically that we are using a plastic enclosure (as apposed to a metal enclosure), so any radiation will not be contained by it. I assume (by visual inspection of the part) that the inductors on the TPS82130 are not of a shielded type? We would also be interested to know if TI would recommend a lower EMI solution to the TPS82130?

I have attached a PDF of the Gerbers, let me know if you need anymore information?SmartBox_PCB.PDF

Which IC you suspected, U7 or U8?

You may check the switching node waveform with >500M Hz Bandwidth scope to check if the ring of SW node is close to 30MHz, which could be helpful to confirm the noise is from 82130, actually you also see the inductor of 82130 is chip inductor with shielding, and we haven't received similar EMI case caused by 82130 yet.

I will also check PCB file tomorrow.

Yuchang

ok thanks for confirming that the inductor is indeed a shielded type. I have a 500Mhz scope so can check the SW node ringing frequency. The IC we mainly suspected was U3, so the one set for 3.3V (P3V3D net).

Hi Sean,

Yuchang will give you feedback soon!

BRs

Zixu

Layout of both look ok, switching node waveform would be helpful for confirmation and also you can add shielding of U3 and then check if the RE result will be improved.

Hi Yuchang, please see a scope trace of the switch node waveform attached. It looks like the frequency is ~10.4MHz (1/96ns)? I guess we could be seeing the third harmonic of this, or would you expect it to be the fundamental as mainly sinusoidal?

Bench testing does seem to suggest screening would help, but this is always a bit subjective compared to testing in an anechoic chamber.

tek00025.tif

How about frequency of OS/US ring?

That looks to be about a 6ns period, so ~166MHz, so probably too fast for what we are having issues with?

Seems frequency are not match with 30M...

The CE test has been passed, right? suggest add shielding to U3/U7 to confirm if the radiation is from 82130. 

What do you mean by the 'CE test has been passed right?' ..... we are currently working to achieve CE mark, so we have just done a pre-compliance test which has shown up this issue.

Would TI recommend another part which is designed to be lower EMI e.g. the 

?

I mean conductive emissions which shows the low frequency performance.

TPS62933 and TPS62913, TPSM82913 are all with spread spectrum, helpful for EMI improvement.

Yuchang