Found this on M/B forums,
Okay, here's what everyone needs to keep in mind.
Set aside the 3900x for a moment.
3800x, 3700x, and below will have basically the same VRM requirements as older Zen products. What I mean to say is . . . core count has a lot to do with how the total power demand for the chip breaks down between voltage and current. More cores = higher current draw, and drawing more current puts more stress on the VRMs. So if you are on 8c today, then the stress to the VRMs within a given power envelope remains the same as it was two years ago. If you are trying to push a 105w 8c Matisse, the current demands are about the same as a 105w Summit Ridge. At least until we bring in AVX2 but let's not get too far out there.
More current draw tends to heat up VRMs. VRMs have a per-phase current limit in their specs. Sometimes mobo manufactuerers are nice enough to tell you that limit. You don't want your VRMs getting close to their current limits, because that makes them run hot, and if they get over 90-100C or higher they can cook. That shortens the life of the board. See overclocked R7 1700x on cheap B350 boards, etc.
The best VRM layout on X370 was the X370 Taichi, with a . . .12+4 setup (6+2 with doubler). They're 60-amp rated. You won't find many VRM configs better with X570 except maybe on one of the MSI boards, I don't know. They might go crazy with more than 16 phases. It looks like the top X570 Asus boards have a similar config to the Taichi (which is far better than what the Crosshair VI Hero had in 2017). X370 Taichi can push over 300W out of the socket. I would imagine that the X570 boards with good 12+4 configs will do about the same . . . maybe more.
So to summarize, if you have an 8c Matisse, you want about the same VRM config that you used on Summit Ridge or Pinnacle Ridge. And looking at X570, it looks to me like anything 8+2 will suffice for regular operation or maybe even mild overclocks. 12+2 will be better because 4 more phases for the CPU. 12+4 gives you an extra two phases for SoC functions/RAM/memory controller. Er, I think.
I would not go with anything less than 8+2 config for an 8c. Fortunately, everything X570 seems to have 8+2 as a minimum (at least in Asus' lineup; you'll have to examine other OEMs to see if they do the same).
3900x is going to tilt its current draw by up to +50% compared to the 3800x within a given power envelope. That's gonna heat up those 8+2 configs quickly. You might be able to get away with it if you stay within the 105w TDP strictly, but any kind of PBO/XFR/overclocking and watch out. You need to spread that current draw out among as many phases as possible. 12+2 or 12+4 configs are preferred for this chip. Eventual 16c Matisse will be even more hardcore.
Also, not all VRMs are made the same. AM4 boards of the past have not only had some low-phase configs like 4+3 and 4+2, but they've also had VRMs with current limits lower than 60 amps. That's bad juju for high-core-count CPUs. Look to someone like Buildzoid (or similar) to give you the skinny on what kind of VRMs are in use on any given board. If you are seeing current limit per phase below 60 amps, you might want to steer clear of that board.