#002

In this video we use 2 Texas Instruments switched-mode power supply development boards to evaluate the importance of good layout and schematic design for keeping conducted emissions down.
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[@kamatihasheela2995]

WOW... this is trully gold in terms of information on this top. this was very helpful to me as an electronics systems designer!

[@andyeadie7946]

Glad to know the info is being found useful!

[@brainfleming8756]

Thanks as always!

[@shazam6274]

Much of the > 100 MHz noise is from the switching rectifier, where the snubber helps a lot. The better solution for these types of low power (2-3 A) designs is a Buck regulator with a Synchronous rectifier, available from TI, MPS, and many others.
Also from 0.5 to 2 MHz are the AM broadcast bands. A switching supply operating near or at these frequencies is a real problem. Both the Vendors mentioned have ICs operating at or above 2 MHz, which mostly solve this issue.
Most vehicle manufacturers specify CISPR 25, Level 5, with their own lowered limits, often less than 25 uV. For conducted emissions below 0.5 MHZ for vehicles, the only real solution is a large input inductor filter.
I used to design audio power amplifiers > 400 W into 2 Ohms for OEM vehicle manufacturers which could draw >40 A from 12V.
If it doesn't pass conducted emissions, it wont pass radiated emissions.

[@EMCFastPass]

I might do another video with the snubber removed to see the effect. Thanks for your input.

[@kesharrawal7724]

Worth watching,

[@Graham_Wideman]

6:30 "pretty decent loop antenna structure for efficiently converting conducted noise into a magnetic field but by moving the sense resistor to the other side of the inductor the rate of change of voltage and current is now much slower which limits the ability of the resistor to radiate" - I'm a bit puzzled by this remark. The inductor and resistor are in series, so surely the rate of change of current is the same in both components, and also the same in both design "A" and "B"? I do realize that with the resistor before the choke it will see a higher AC voltage than if it's after the choke (in which case quite low VAC). Do you think that actually the contrast in VAC makes the significant radiation difference?
I suggest a couple of alternative rationales for moving the resistor loop: In the "A" position: In version "A", the large VAC meant significant parasitic capacitive loading when attaching a current clamp at that location (or a differential probe for that matter), which might have made a noticeable change in the behavior of the switching circuit -- it's tantamount to adding a snubber at that location. Having noticed that in the A version, they were motivated to swap the component order for the "B" version, so as to be able to take the same measurement, but without disturbing the switching operation.
Or it could be just that using a differential probe across the resistor in the "A" design subjected the probe to relatively high AC voltages, in which, aside from possible probe damage, most of the AC voltage is common mode and not the voltage drop corresponding to the desired current measurement, in turn requiring a high CMRR in the probes for an accurate current reading. Whereas that's not a concern in the "B" design, where the VAC is far smaller, but the different voltage reflecting current is the same as before, so a far higher signal to CM ratio. (And is also simpler to deal with as a teaching tool.) Or maybe "all of the above". Hahaha. Thoughts?
Also I concur with other viewers that it's great to see videos that dig into topics like this in some depth! Very much appreciated!

[@rjordans]

I would expect that the biggest difference from moving the resistor is in reducing the capacitance in the high voltage swing region which helps keeping the ringing down

[@artz0529]

Thank you sir

[@rjordans]

Another thing I've been doing in my designs is not using a filled polygon for the switching net. It's just a small length of trace so current carrying capabilities are typically not a problem but reducing the capacitance seems to help keeping ringing down

[@PafiTheOne]

I found interesting how the low frequency performance made worse by the improvement. :-) I suspect the increased switching current caused by the snubber is responsible for it.

[@EMCFastPass]

I might do another video with the snubber removed to see the effect. Thanks for your input.

[@paridhigupta1770]

@@EMCFastPass Hi Andie, please share findings with the snubber circuit removed.

[@k7iq]

I have a product (charger) where the resistor in series with the capacitor placed across switching FETs breaks and I believe it causes quite a bit of EMI. Oooops. Gotta pick a bit larger resistor I think.

[@k7iq]

This is great but why would you want to use one of those VERY EMI noisy bench supplies too ?? Those are awful. Just saying
:)

[@EMCFastPass]

I know right? Time to get a new one I think!

[@k7iq]

Well, it looks like it worked just fine for this test and probably most conducted tests with the LISN. I just notice that similar power supplies are really noisy. Mastech, Dr. Meter and others that are more likely manufactured by the same company in China. They also seem to break a bit too often but are sometimes fixable at least. Not always though.
:)