EMC test for SMPS without spectrum analyser

[eem2am]

smps spectrum

Hello,

I have designed and built a mains powered, switch-mode LED lamp (15W).

I want to get an idea of how it is going to perform in the conducted emissions EMC test.
However, I do not have a spectrum analyser.

I do though, have a scope with 20MHz bandwidth.

I was thinking, if I put small value current sense resistors in the Live and Neutral lines…..then if I scope across these resistors……..surely, would this give me a good idea of the conducted emissions ?

And would I be right in saying that if each current sense resistor is devoid of switching frequency ripples then its going to pass the differential mode emissions test?

Also, if the magnitude and shape of the waveforms across each current sense resistor are the same, then does that mean that it will pass its common mode emissions test?

Thankyou for reading.

 

[FvM]

how to test smps

Regular conducted emission measurement is voltage across an equivalent circuit of grid impedance, so a current measurement isn't exactly the same. But it can give a rough estimation of emission spectrum and intensity.

Generally, an oscilloscope current probe is a good tool. A selfmade current transformer (e.g. a torroid core with a suitable number of secondary windings and load resistor) is better than shunts in several regards: It provides isolation, prevents from unintended ground loops and allows differential and common mode measurements with the same device by simply changing the primary.

 

[eem2am]

how to test smps

Thankyou FvM, I am grateful for your help.

Do you think this set up would be OK for the common mode conducted emissions test?

..And this set-up for the differential mode conducted emissions?...

When you speak of ground loops with the series resistor method that i suggested in the first post, do you mean loops through the oscilloscope probe through ground?

Thankyou for reading.

 

[FvM]

emc emission smps

Yes, you read my mind in all details.