I have recently designed an audio player that uses the TI TPA3112D1 power amplifier, and mostly based it on the reference design and schematic. I find however, that the design is failing the conducted emissions test due to noise feeding back into the 24VDC supply. This noise starts at 340khz with harmonics present proceeding up the freqency range. I assume this to be from the amplifier, as the noise is starts as soon as the amp is unmuted. The noise is present regardless of audio level output of the amplifier.
I have tried adding different types and variations of bypass caps, which seems to have really no effect. I also tried a 12uh inductor (AISC-1210HS-120K-T2) I had on hand on the positive supply input, which also made no noticeable difference. The test house also tried a Steward LFB310190-000, which made little difference.
The PCB is a 6 layer board, the pictures below don't include layers 3 and 4, as there are no traces or planes close to the power and power amp circuit.
The power circuit uses a common mode choke, which probably doesn't target these low freqencies at all.
Googling this is largely ineffective, as most app notes are geared towards emi on the output side of a Class D amp. It may be I need low freqency filtering of some sort, but it isn't clear to me.
My own lab results
Pre and Post 12 uH inductor, prior to common mode choke
24V + and -, with A-B, at output, through common mode choke(It would almost appear to amplify these signals!?)
Attempted fix 1
In this fix, I have cut the traces between the via and the sm caps, routing the supply through the cap leads. This cap is a 470uf 35v electrolytic. I have also double stacked 1nf and .1uf on each side.
L4 and L5 are part HI0805R800R-10
C29 and C13 1nf 50v are part C0402C102K5RACTU
C30 and C31 0.1uf 50v are part C0603C104K5RACTU
Results are no better, probably slightly worse. The most noticeable difference is caused by touching output leads or speaker body, which causes these spikes/harmonics to approximately double in amplitude. This would seem to indicate that output filtering (L4 and L5) play a part in how this works.