Hi,
Working with PoE for the first time using the TPS23754 in a design that matches the -420EVM. I got my first board spin in and although I did get 5v on my output which could easily handle a 2A load (all my system requires), the output was very noisy regardless of my load. I had roughly 1.4v ripple at about 13kHz to the point where it is quite audible. I chased the noise through the system and it basically exists everywhere as soon as I get into my RTN ground section. I can however, cause the noise to vary and in some cases go away momentarily by bridging my RTN and digital grounds with my finger near the feedback section.
After discussing my board's behavior with my FAE, we reviewed my schematic and found it to match the EVM, however my layout was fairly different due to form factor constraints. In particular, I had swung some of my feedback circuitry, including my digital ground plane back over near the input side of the TPS23754 and my thought was that maybe there was some noise coupling in there since on the EVM, those components are all on the power side of the flyback transformer.
Luckily, my board constraints were relaxed a bit by that point and I was able to do another spin of the board which was more true to the EVM layout (not exactly the same but the flow is very close). Also, a number of principles in the EMI compliance design note had been overlooked so those techniques were implemented.
I received the boards earlier this week, and this time had a technician hand assemble instead of getting pick and placed at our CM. He saw no issues between my BOM and the parts that we had used to populate the boards so I'm fairly confident both revs have parts that match my design. Anyways, I fired up the new boards and the result is still the same.
Any thoughts on areas that I can probe to find where the source of this noise could be coming from. I actually see the 13kHz in a number of areas including on my Gate pin where normally on the EVM I see a 250kHz pulse train, on my board I see 250kHz pulses and then a long gap on the last pulse which repeats roughly every 13kHz (with some variation). It's also on the input to my flyback transformer as well, not surprisingly.
In the meantime, I'll be double checking that my BOM matches that of the EVM. I can post excerpts of the schematic and layout if that might be helpful. Screen shots of the output signal and my gate pin are attached.
Thanks,
Eric
UUT Vout signal:
UUT Gate Signal (purple) compared to EVM Gate(white):
