TPS23754: Radiated switching noise

Borge Strand-Bergesen

Part Number: TPS23754
Other Parts Discussed in Thread: , TPS23755, TPS23758

Hi,

I experience high levels of switching noise from my TPS23754 design. I have copied the TPS23754EVM-383 reference design and loaded both it and my own design with a 36R resistors for 4W power consumption at the 12V output.

I use an RF current monitor probe around the ethernet cable. My spectrum analyzer is able to give good relative measurements up to 40 MHz. Both boards show narrow-band noise at the primary switching frequency (242.65kHz ref design, 260.5kHz my board) and at multiple harmonics.

Please help me out with these questions:
- In your experience, will the TPS23754EVM-383 on its own pass FCC and CE certification for PoE devices consuming some 20W? If the TPS23754EVM-383 passes, I believe my design passes as well.
- What action should I take to reduce conducted switching noise? (Common-mode choke, snubbers, tuning LC decoupling, other action)

Thanks,

Borge

over 5 years ago

0 Michael Pahl over 5 years ago

TI__Mastermind 21320 points

Hello Borge,

So officially from the datasheet, " This evaluation board/kit does not fall within the scope of the European Union directives regarding electromagnetic compatibility, restricted substances (RoHS), recycling (WEEE), FCC, CE or UL, and therefore may not meet the technical requirements of these directives or other related directives." (pg 14.)

Furthermore, we have an FCC Warning that reads, "FCC Warning: This evaluation board/kit is intended for use for ENGINEERING DEVELOPMENT, DEMONSTRATION, OR EVALUATION PURPOSES ONLY and is not considered by TI to be a finished end-product fit for general consumer use. It generates, uses, and can radiate radio frequency energy and has not been tested for compliance with the limits of computing devices pursuant to part 15 of FCC rules, which are designed to provide reasonable protection against radio frequency interference. Operation of this equipment in other environments may cause interference with radio communications, in which case the user at his own expense will be required to take whatever measures may be required to correct this interference."

When I read the FCC and CE standards and definitions, it comes off to me like a system level requirement. I checked my desk phone and my laptop, and they have those certifications. So I believe this is meant for a system level certification versus an evaluation board.

As for the actions to take, all of the ones you mentioned ( Common-mode choke, snubbers, tuning LC decoupling, other action) are all steps I would recommend for reducing EMI noise. If you are still in the evaluation phase, you could consider using one of our parts that has Spread Spectrum Frequency Dithering, which will also help.

If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

Regards,

Michael P.

Applications Engineer

Texas Instruments

0 Borge Strand-Bergesen over 5 years ago in reply to Michael Pahl

Intellectual 710 points

Hi Michael,

Thanks for your response!

Please let me know your spread spectrum suggestions. As for the other tweaks, any details would be appreciated for which may be best suited to improve on the performance of the dev kit and its derivatives.

I'll be throwing my Quasimodo at snubber development, and a vector network analyzer at various impedance tweaks.

Best,

Børge

0 Michael Pahl over 5 years ago in reply to Borge Strand-Bergesen

TI__Mastermind 21320 points

Hello Borge,

Currently our TPS23755 and TPS23758 (type 1 PDs) have this feature. We can discuss more options offline.

As for other suggestions, this following app note has suggestions for reducing EMI in an isolated PoE system:

http://www.ti.com/lit/an/slua469/slua469.pdf

Using the suggestions you said, honestly the best advice is to test the device in the lab and change parts as needed.